feat: the artist picker on Patreon and SubscribeStar too — and the Patreon name is canon (milestone 429)

Operator: "yes add the artist picker to patreon and subscribestar too. but
generally we treat the patreon name as the canon"

- Every add now goes through the panel. On Patreon/SubscribeStar it opens
  on the creator's display name (read only when the panel opens:
  probe?names=1), searches FC's artists with it, and auto-picks a match.
  An untouched URL handle sends no name, so the server still resolves it.
- Patreon is canon: joining a Patreon source to an artist known by another
  name offers "Rename “x” to the Patreon name “X”", ticked by default.
  quick-add's use_platform_name renames server-side from the name it reads
  itself; name only, the slug never moves (#130); never to a URL handle when
  the name can't be read; ignored on SubscribeStar and Discord.
- _platform_display_name returns None rather than the handle, bounded by the
  same 6s lookup budget as Discord's names.
- panelDefaults / addRequest / renameOffer replace the Discord-only helpers.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01LVjrnpQjRgHdvq95rASoiR

extension/background/background.js

@@ -270,6 +270,7 @@ browser.runtime.onMessage.addListener(async (msg) => {
         return await api.quickAddSource(msg.url, {
           artistId: msg.artistId ?? null,
           artistName: msg.artistName ?? null,
+          usePlatformName: msg.usePlatformName === true,
         });
       } catch (e) {
         return { error: e.message };
@@ -284,7 +285,7 @@ browser.runtime.onMessage.addListener(async (msg) => {
 
     case 'PROBE_SOURCE':
       try {
-        return await api.probeSource(msg.url);
+        return await api.probeSource(msg.url, { names: msg.names === true });
       } catch (e) {
         return { error: e.message };
       }