fix: MediaProjection权限丢失后无法回退无障碍截图和Web画面闪烁

- 采集循环每帧检测mediaProjection对象有效性，null时立即回退无障碍截图
- 统一fallbackToAccessibilityCapture方法，消除7处重复回退代码
- 无障碍截图失败后等待MIN_CAPTURE_INTERVAL剩余时间，避免截图间隔太短错误
- Web端canvas尺寸锁定策略：首帧锁定尺寸，后续帧统一缩放绘制，消除闪烁

app/src/main/java/com/hikoncont/manager/ScreenCaptureManager.kt

@@ -435,8 +435,7 @@ class ScreenCaptureManager(private val service: AccessibilityRemoteService) {
                 } else {
                     // MediaProjection 不可用，回退到无障碍截图
                     Log.i(TAG, "Android 11+：MediaProjection 不可用，回退到无障碍截图模式")
-                    enableAccessibilityScreenshotMode()
+                    fallbackToAccessibilityCapture()
-                    startAccessibilityScreenCapture()
                 }
             } else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
                 Log.i(TAG, "Android 5-10：使用 MediaProjection VirtualDisplay 连续流式捕获")
@@ -464,13 +463,7 @@ class ScreenCaptureManager(private val service: AccessibilityRemoteService) {
             // 先确保 MediaProjection 可用
             if (!ensureMediaProjection()) {
                 Log.e(TAG, "MediaProjection 不可用，回退到无障碍截图")
-                // 回退到无障碍截图作为兜底
+                fallbackToAccessibilityCapture()
-                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
-                    enableAccessibilityScreenshotMode()
-                    startAccessibilityScreenCapture()
-                } else {
-                    startFallbackCapture()
-                }
                 return@launch
             }
             
@@ -481,12 +474,7 @@ class ScreenCaptureManager(private val service: AccessibilityRemoteService) {
             
             if (virtualDisplay == null) {
                 Log.e(TAG, "VirtualDisplay 创建失败，回退到无障碍截图")
-                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
+                fallbackToAccessibilityCapture()
-                    enableAccessibilityScreenshotMode()
-                    startAccessibilityScreenCapture()
-                } else {
-                    startFallbackCapture()
-                }
                 return@launch
             }
             
@@ -505,18 +493,22 @@ class ScreenCaptureManager(private val service: AccessibilityRemoteService) {
             
             while (isCapturing) {
                 try {
+                    // MediaProjection对象有效性检测：
+                    // onStop回调会将Holder中的对象清理为null，
+                    // 此时继续采集毫无意义，应立即回退到无障碍截图
+                    if (mediaProjection == null) {
+                        Log.w(TAG, "MediaProjection对象已失效(被系统回收或onStop清理)，立即回退到无障碍截图")
+                        cleanupVirtualDisplayOnly()
+                        fallbackToAccessibilityCapture()
+                        return@launch
+                    }
+                    
                     // Surface有效性检测：BufferQueue被系统abandoned后立即回退
-                    // 防止acquireLatestImage()持续返回null触发无限重建循环
                     val currentSurface = imageReader?.surface
                     if (currentSurface == null || !currentSurface.isValid) {
-                        Log.w(TAG, "ImageReader Surface已失效(null=${currentSurface == null}, isValid=${currentSurface?.isValid}), 停止采集并回退到无障碍截图")
+                        Log.w(TAG, "ImageReader Surface已失效(null=${currentSurface == null}, isValid=${currentSurface?.isValid}), 回退到无障碍截图")
                         cleanupVirtualDisplayOnly()
-                        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
+                        fallbackToAccessibilityCapture()
-                            enableAccessibilityScreenshotMode()
-                            startAccessibilityScreenCapture()
-                        } else {
-                            startFallbackCapture()
-                        }
                         return@launch
                     }
                     
@@ -559,12 +551,7 @@ class ScreenCaptureManager(private val service: AccessibilityRemoteService) {
                                         Log.w(TAG, "MediaProjection连续${consecutiveBlackFrames}个黑屏帧，切换到无障碍截图模式")
                                         safeRecycleBitmap(bitmap)
                                         cleanupVirtualDisplayOnly()
-                                        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
+                                        fallbackToAccessibilityCapture()
-                                            enableAccessibilityScreenshotMode()
-                                            startAccessibilityScreenCapture()
-                                        } else {
-                                            startFallbackCapture()
-                                        }
                                         return@launch
                                     }
                                     
@@ -600,12 +587,7 @@ class ScreenCaptureManager(private val service: AccessibilityRemoteService) {
                             if (virtualDisplayRebuildCount > MAX_VIRTUAL_DISPLAY_REBUILD_ATTEMPTS) {
                                 Log.w(TAG, "重建次数已达上限，回退到无障碍截图")
                                 cleanupVirtualDisplayOnly()
-                                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
+                                fallbackToAccessibilityCapture()
-                                    enableAccessibilityScreenshotMode()
-                                    startAccessibilityScreenCapture()
-                                } else {
-                                    startFallbackCapture()
-                                }
                                 return@launch
                             }
                             
@@ -619,10 +601,7 @@ class ScreenCaptureManager(private val service: AccessibilityRemoteService) {
                             if (virtualDisplay == null || rebuiltSurface == null || !rebuiltSurface.isValid) {
                                 Log.w(TAG, "VirtualDisplay 重建后无效(vd=${virtualDisplay != null}, surface=${rebuiltSurface != null}, valid=${rebuiltSurface?.isValid}), 回退到无障碍截图")
                                 cleanupVirtualDisplayOnly()
-                                if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
+                                fallbackToAccessibilityCapture()
-                                    enableAccessibilityScreenshotMode()
-                                    startAccessibilityScreenCapture()
-                                }
                                 return@launch
                             }
                         }
@@ -642,12 +621,25 @@ class ScreenCaptureManager(private val service: AccessibilityRemoteService) {
         }
     }
     
+    /**
+     * 统一回退到无障碍截图的方法
+     * Android 11+ 使用无障碍截图，低版本使用测试图像兜底
+     */
+    private fun fallbackToAccessibilityCapture() {
+        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
+            enableAccessibilityScreenshotMode()
+            startAccessibilityScreenCapture()
+        } else {
+            startFallbackCapture()
+        }
+    }
+    
     /**
      * 确保 MediaProjection 可用
      * 
      * 核心修复：禁止重复调用 getMediaProjection(resultCode, resultData)
      * 每次调用都会创建新实例，系统会自动 stop 旧实例，触发 onStop 回调，
-     * 形成"权限丢失→恢复→再丢失"的死循环，这是权限频繁掉落的根因。
+     * 形成"权限丢失->恢复->再丢失"的死循环，这是权限频繁掉落的根因。
      * 
      * 只从 MediaProjectionHolder 获取已有对象，不重新创建。
      */
@@ -727,11 +719,19 @@ class ScreenCaptureManager(private val service: AccessibilityRemoteService) {
                         }
                     }
                     
-                    // 截图成功时按帧率延迟，失败时短延迟后立即重试
+                    // 截图成功时按帧率延迟，失败时等待最小截图间隔后重试
                     if (consecutiveFailures == 0) {
                         delay(1000 / dynamicFps.toLong())
                     } else {
-                        delay(50) // 失败时50ms后重试，让系统截图间隔限制自己控制节奏
+                        // 无障碍截图有系统级最小间隔限制(约3秒)，
+                        // 失败时等待剩余间隔时间，避免"截图间隔太短"错误
+                        val elapsed = System.currentTimeMillis() - lastScreenshotTime
+                        val remaining = MIN_CAPTURE_INTERVAL - elapsed
+                        if (remaining > 0) {
+                            delay(remaining)
+                        } else {
+                            delay(100)
+                        }
                     }
                     
                 } catch (e: CancellationException) {