feat(career): profile passport wall, home career card, shareable PNG card (#955)

* feat(career): profile passport wall, home career card, shareable PNG card

Career v3, WS2. The identity artifact leaves the plugin tab:

- Profile: #v3-profile-passports-mount (core, one div) filled by career
  on v3:profile-rendered — per-instrument shelves of earned covers,
  hours, gig count, open-career link. Absent-not-empty.
- Home: the plugin-count stat tile becomes #v3-dash-career-slot with the
  old stat as fallback content; career replaces it with a trading-card
  tile (leather + foil shine, badge count, hours, closest-stamp ask) on
  the existing v3:dashboard-rendered event.
- Shareable card: static/js/blob-io.js (downloadBlob lifts the idiom
  duplicated verbatim in settings-io/diagnostics-export — both
  refactored; copyImageBlob wraps ClipboardItem, returns false to signal
  the download fallback). Earned passports get Save/Copy card: a
  natively-drawn 480×640 canvas (leather, stamp ring, stubs+hours line);
  copy falls back to download with a notice when the clipboard refuses.
- Mount-point convention documented in docs/plugin-v3-ui.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

* fix(career): external surfaces stay absent until a passport exists

CodeRabbit on #955: a bare commitment produced a zero-passport wall and
replaced the dashboard fallback. Docs also now say mounts may hold
fallback content and plugins REPLACE, never append.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

static/js/diagnostics-export.js

@@ -21,6 +21,8 @@
 //      redact toggles.
 //   3. Stream the returned zip to disk.
 
+import { downloadBlob } from './blob-io.js';
+
 function _diagIncludeFromUI() {
     const v = (id) => document.getElementById(id)?.checked !== false;
     return {
@@ -265,14 +267,7 @@ export async function exportDiagnostics() {
     }
     try {
         const blob = await resp.blob();
-        const url = URL.createObjectURL(blob);
-        const a = document.createElement('a');
-        a.href = url;
-        a.download = filename;
-        document.body.appendChild(a);
-        a.click();
-        document.body.removeChild(a);
-        URL.revokeObjectURL(url);
+        downloadBlob(blob, filename);
         status.textContent = `Exported ${filename}`;
     } catch (e) {
         status.textContent = `Export failed during download: ${e.message}`;