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>

README.md

@@ -26,10 +26,12 @@ service/      tiny Node.js resolver (one file, zero npm deps)
   resolver.js
   package.json
   .env.example
-extension/    Firefox MV2 extension (content script only)
+extension/    Firefox MV2 extension
   manifest.json
+  background.js   (does the resolver fetch; talks to the content script)
   content.js
   styles.css
+  icon.svg
 ```
 
 ## 1. Run the resolver service
@@ -91,9 +93,26 @@ with `xpinstall.signatures.required = false`.)
 
 1. Make sure the resolver service is running.
 2. Open any Reddit comments thread (a URL with `/comments/` in it).
-3. Artist names in the comments become links: the name links to **Spotify**, with
-   a small superscript **`bc`** link to **Bandcamp** beside it. New comments loaded
-   as you scroll are linked automatically.
+3. Artist names become a single link — to **Spotify** if the artist is on Spotify,
+   otherwise to **Bandcamp** — colour-coded (green = Spotify, teal = Bandcamp). New
+   comments loaded as you scroll are linked automatically.
+4. The toolbar icon shows the extension is active; its badge shows how many links were
+   added on the current page.
+
+## Troubleshooting
+
+If nothing gets linked:
+
+1. Confirm the resolver is running — `curl localhost:8787/health` should return
+   `{"ok":true,...}`.
+2. On the reddit thread, open the browser console (F12) and look for `[rsl]` logs: they
+   report the root element, candidate count, and resolver responses on each scan. A
+   `resolve failed` line means the background script can't reach the resolver (is it on
+   `:8787`?); `candidates found: 0` means the comment text wasn't matched (please report
+   the thread).
+3. The content script reaches the resolver **through the extension's background script**
+   — a page-context fetch to `http://localhost` is blocked by reddit's CSP — so make sure
+   the add-on loaded without errors in `about:debugging`.
 
 ## Configuration