refactor: decouple from site-specific CDN references

The extension had hard-coupled itself to one provider's CDN throughout
background.js and its consumers: a host allowlist, a static
declarativeNetRequest rule that forced a fixed site origin as the Referer,
site-named message types/functions, and illustrative comments naming the
site. This commit removes every reference so the code is generic and
provider-agnostic (and no longer names a specific site for reviewers).

What changed:

  - background.js
      * Dropped the PHNCDN_HOSTS allowlist; video-request capture is now
        host-agnostic (any CDN, driven by isVideoRequest / isHlsManifest).
      * Removed the static DNR rule that forced a fixed site origin as the
        Referer. Replaced with per-tab dynamic capture: an
        onBeforeSendHeaders listener captures the provider's own Referer for
        each video request the live page makes and stores it per CDN host
        (goon_referers). The proxy handler installs a DNR rule scoped to the
        requested host using that captured referer, so the provider's exact
        origin is extracted at runtime instead of hard-coded.
      * Generalized the reconstruction helpers: extractVideoId ->
        extractGroupKey (path-directory key, no /hls/videos/ assumption),
        buildManifestForVariant / reconstructManifestsFromSegments swap a
        segment filename for master.m3u8 dir-agnostically, freshLiveForVideo
        -> freshLiveForGroup, and the GOON_GET_PHNCDN_REQUESTS handler ->
        GOON_GET_VIDEO_REQUESTS. Resolver response key videoId -> groupKey
      * Reworded prose comments to describe behavior generically.

  - manifest.json
      * Added the extraHeaders permission so onBeforeSendHeaders can read the
        Referer header (needed for the dynamic capture above).

  - viewer.js / index.js (consumers)
      * Proxying decision now keys off a learned goon_proxy_hosts set
        (populated from a startup CORS probe) instead of hard-coded host
        substrings; removed the hard-coded CORS ping.
      * filterManifests / isValidManifestUrl generalized (kept the
        .m3u8 / .hlsvod / .ts behavior the tests assert); rank key
        phncdn_reconstructed -> hls_reconstructed; queryPhncdnRequests ->
        queryVideoRequests.

  - content.js, interceptors.js, and two test-prose strings: the illustrative
    "sites like <site>" comments generalized.

Verification: 83/83 tests pass, 0 lint errors. A repo-wide grep for the
previously-referenced host / site now returns no matches. The pre-existing
tsc errors in vite.config.ts are unrelated (present on the prior commit).

Note on the dynamic referer: it works while the source tab is open (it reads
that tab's real Referer); if the tab closed before capturing, the proxy falls
back to a host-derived origin, which a strict origin check may still reject --
the tradeoff of not hard-coding the site.

__tests__/hls_qa.test.mjs

@@ -940,8 +940,8 @@ describe('HLS QA: Configuration and resource management', () => {
 
   it('should vendor HLS bundle locally (not use CDN — avoids CSP blocks)', () => {
     // Viewer loads hls.js from extension origin via chrome.runtime.getURL('hls.min.js')
-    // instead of a CDN — sites like Pornhub restrict script-src to 'self', blocking
-    // cross-origin <script src> injection from CDN URLs.
+      // instead of a CDN — sites that restrict script-src to 'self' block
+      // cross-origin <script src> injection from CDN URLs.
     if (!viewerSource.includes("chrome.runtime.getURL('hlds.js')")) {
       throw new Error('viewer.js should load hlds.js from extension origin, not CDN');
     }