fix: background-script fetch (CSP/mixed-content), prefer-Spotify links, toolbar icon (task/fix-extension-and-linkui)

The extension linked nothing and the resolver saw no requests: the content script
fetched http://localhost:8787 directly from the HTTPS reddit page, which reddit's CSP /
mixed-content blocks. Move the network call to a background script (extension origin,
exempt) and add stage logging so failures are visible.

- extension/background.js (new): does the resolver fetch on message; sets the toolbar badge.
- extension/content.js: talks to background via runtime messaging (no page-context fetch);
  [rsl] console logs at boot/root/candidates/resolve/scan; single primary link.
- extension/manifest.json: background.scripts + browser_action (icon.svg, title).
- extension/{styles.css,icon.svg}: platform-coloured link (green=spotify, teal=bandcamp) + toolbar icon.
- service/lib.js: primaryOf()/isSearchUrl() — link Spotify when the artist is on Spotify,
  else Bandcamp, else Spotify search; spotifyResult/mbResult attach `primary` (unit-tested).
- README: link behaviour, toolbar, Troubleshooting.

Verified: scripts/check.sh green (10 tests); resolver returns `primary` end-to-end. The
in-browser fix is logic-verified only (no headless Firefox here) — reload the temp add-on
and watch the page console `[rsl]` logs to confirm.

Source: project human-feedback 2026-06-16 (notes 01KV8VC58QMYBDY6VVVJHR8YNY + 01KV91AMJCN2C4GFCKMKQRGNXE).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

extension/styles.css

@@ -1,23 +1,16 @@
-/* Injected links — kept subtle so reddit comments stay readable. */
+/* Injected links — kept subtle so reddit comments stay readable. One link per artist,
+   coloured by platform: green = Spotify, teal = Bandcamp. */
 .rsl-link {
-  text-decoration: underline dotted #1db954 !important; /* spotify green */
+  text-decoration: underline dotted;
   text-underline-offset: 2px;
   cursor: pointer;
 }
+.rsl-spotify {
+  text-decoration-color: #1db954 !important; /* spotify green */
+}
+.rsl-bandcamp {
+  text-decoration-color: #629aa9 !important; /* bandcamp teal */
+}
 .rsl-link:hover {
-  text-decoration: underline solid #1db954 !important;
-}
-.rsl-sup {
-  font-size: 0.7em;
-  margin-left: 1px;
-  vertical-align: super;
-  line-height: 0;
-}
-.rsl-bc {
-  color: #629aa9 !important; /* bandcamp teal */
-  text-decoration: none !important;
-  font-weight: 600;
-}
-.rsl-bc:hover {
-  text-decoration: underline !important;
+  text-decoration-style: solid !important;
 }