web: three panels — Generate, Gallery, Collections — and a layout

ported from the standalone app's hash-tab UI. The panels do not coordinate through props (the panel
contract only ever hands a component its panelId) or usePanelChannel-as-selection:

- Gallery's lightbox is ?selected=<id>, Collections' detail is ?collection=<id> — different names
  since both panels render on the same route at once
- Generate bumps useAtelierRefresh (added to officerdev/src/channels.ts) when a job finishes;
  Gallery subscribes to it — a genuine signal, not a selection, so it stays a channel

images render through GET /files/:filename with a ?token= query param appended client-side
(lib/api.ts's fileUrl), the same escape hatch FileViewer and the music plugin use for exactly this:
a browser cannot set an Authorization header on an <img src>.

Providers is not here — see PLUGIN.md for why it became a platform settings screen instead.

assets/icon.png and assets/splash.png are [placeholder] flat swatches, not real artwork — see
assets/PLACEHOLDER.md.

assets/PLACEHOLDER.md

@@ -0,0 +1,9 @@
+# [placeholder] artwork
+
+`icon.png` (256×256) and `splash.png` (1024×1024) are flat `#f59e0b` swatches — the manifest's accent
+colour, nothing more. No real Atelier artwork existed to carry over from the standalone service, and
+drawing one is outside an unattended run's scope.
+
+Replace both with real artwork before this plugin is published to the Marketplace; until then the dock
+tile and any mobile launch screen will show a plain amber square. Grep for `[placeholder]` across this
+plugin to find every other invented value.