refactor: audit fixes — clean up bugs, dead code, and type mismatches

- viewer.js: remove undefined state.segmentsBuffered assignment in BUFFER_RESET
- viewer.js: add missing 'switching' and 'ended' phases to PHASE_ICONS
- index.js: remove redundant Strategy B3 (duplicated B1 video-element walk)
- index.js: filter standard <video> DOM attributes from custom-property scans
  (m3u8DeepProbeFn B1 and scrapeFn Priority 3) to avoid iterating built-in
  properties that can never contain HLS data
- background.js: remove unconditional return true (handlers are synchronous)
  and remove dead error-response for unknown message types
- types.ts: align MediaType union ('direct' | 'hls') with actual runtime
  value used in code ('video' | 'hls')
- package.json: remove hls.js from dependencies (loaded from CDN at runtime,
  never imported as an npm module)

src/viewer/viewer.js

@@ -170,7 +170,9 @@ const PHASE_ICONS = {
   loading: '\u2B07',    // down arrow
   buffering: '\u27F7',  // spinning
   playing: '\u25B6',    // play
+  switching: '\u21BB',  // right arrow corner
   error: '\u2717',      // X
+  ended: '\u23F9',      // stop
 };
 
 /**
@@ -413,7 +415,6 @@ function wireHlsEvents(hls, video, panel) {
     state.lastActivity = Date.now();
     state.bufferedAhead = 0;
     state.segmentsLoaded = 0;
-    state.segmentsBuffered = 0;
     renderPanelState(video, panel);
   });