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/settings-io.js

@@ -1,6 +1,6 @@
 // Settings backup — the export / import bundle.
 //
-// Carved verbatim out of static/app.js (R3a). A LEAF module: imports nothing.
+// Carved verbatim out of static/app.js (R3a). Imports only the blob-io leaf.
 //
 // Two entry points, both inline handlers on the Settings screen, so app.js keeps
 // re-exposing them on window. The import is two-phase (server first, atomic; then
@@ -29,6 +29,8 @@
 // phase 2; the localStorage side is best-effort merge after server
 // success. Failures are reported, never silenced.
 
+import { downloadBlob } from './blob-io.js';
+
 export async function exportSettings() {
     const status = document.getElementById('backup-status');
     status.textContent = 'Exporting...';
@@ -66,14 +68,7 @@ export async function exportSettings() {
             if (match) filename = match[1];
         }
         const blob = new Blob([JSON.stringify(bundle, null, 2)], { type: 'application/json' });
-        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: ${e.message}`;