refactor(extension): DRY the web-root transform + cookie-export flow (#161)

Behavior-preserving (extension JS has lint-only CI + your manual test):
- api.webRoot() single-sources the baseUrl→web-root transform (strip trailing
  slash + /api) that was copy-pasted in background.js's self-update check and
  OPEN_ARTIST_PAGE, whose comments even cross-referenced each other.
- exportPlatformCookies(key) shares the extract→verify→upload spine between
  EXPORT_COOKIES (single) and EXPORT_ALL_COOKIES; it returns a structured
  outcome so each caller keeps its own response/skip messages verbatim.
- popup.js mutedNote(text) replaces the "centered muted note" div hand-rolled
  in the platform-loading, sources-loading, and empty-sources renderers.

No version bump — no behavior change, so it rides the next real ext release
rather than forcing an AMO re-sign + reinstall.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01NsmJSQxnNxGgtM5Yz4GAqi

extension/popup/popup.js

@@ -2,6 +2,15 @@ document.addEventListener('DOMContentLoaded', init);
 
 const CONNECTION_TEST_INTERVAL = 2 * 60 * 1000;
 
+// A centered muted note div — the loading / empty state shared by the platform
+// and sources lists.
+function mutedNote(text) {
+  const d = document.createElement('div');
+  d.style.cssText = 'text-align:center;padding:18px;color:var(--on-surface-variant);';
+  d.textContent = text;
+  return d;
+}
+
 async function init() {
   try {
     const cfg = await browser.runtime.sendMessage({ type: 'GET_CONFIG' });
@@ -38,10 +47,7 @@ function showSetupRequired() {
 function showPlatformsLoading() {
   const c = document.getElementById('platforms-list');
   c.textContent = '';
-  const d = document.createElement('div');
-  d.style.cssText = 'text-align:center;padding:18px;color:var(--on-surface-variant);';
-  d.textContent = 'Loading platforms…';
-  c.appendChild(d);
+  c.appendChild(mutedNote('Loading platforms…'));
 }
 
 async function testConnectionIfNeeded() {
@@ -183,10 +189,7 @@ async function exportAllCookies() {
 async function loadSources() {
   const c = document.getElementById('sources-list');
   c.textContent = '';
-  const d = document.createElement('div');
-  d.style.cssText = 'text-align:center;padding:18px;color:var(--on-surface-variant);';
-  d.textContent = 'Loading sources…';
-  c.appendChild(d);
+  c.appendChild(mutedNote('Loading sources…'));
   const r = await browser.runtime.sendMessage({ type: 'LIST_SOURCES' });
   c.textContent = '';
   if (r.error) {
@@ -197,10 +200,7 @@ async function loadSources() {
     return;
   }
   if (!r.sources || r.sources.length === 0) {
-    const empty = document.createElement('div');
-    empty.style.cssText = 'text-align:center;padding:18px;color:var(--on-surface-variant);';
-    empty.textContent = 'No sources yet.';
-    c.appendChild(empty);
+    c.appendChild(mutedNote('No sources yet.'));
     return;
   }
   for (const src of r.sources) c.appendChild(createSourceRow(src));