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)

package.json

@@ -13,9 +13,6 @@
     "test:watch": "vitest",
     "check": "tsc --noEmit"
   },
-  "dependencies": {
-    "hls.js": "^1.5.17"
-  },
   "devDependencies": {
     "ajv": "^8.17.1",
     "ajv-formats": "^3.0.1",