fix: vendor hls.js locally (CSP) and handle null executeScript results

Two fixes addressing kfj001 review #37 on commit b19035a:

1. Vendor hls.js bundle locally — chrome.runtime.getURL('hls.min.js')
   instead of CDN, bypassing site CSP (script-src 'self') blocks.
   Added hls.min.js to vite.config.ts entry points, manifest web_accessible_resources.

2. Handle null entries in executeScript results — destroyed frames
   return null when allFrames:true, causing 'Cannot read properties of
   null (reading 'result')' in probeHlsManifests() and scrapeTab().
   Added null guards before accessing .result.

83/83 tests pass. Build: 10 modules, 413KB hls bundle.

manifest.json

@@ -33,7 +33,7 @@
   ],
   "web_accessible_resources": [
     {
-      "resources": ["interceptors.js"],
+      "resources": ["interceptors.js", "hls.min.js"],
       "matches": ["<all_urls>"]
     }
   ],