feat: lightroom-style workspace UI with album covers

Backend:
- expose cover thumbnail URL on album responses (LEFT JOIN + presign)
- auto-assign cover only when empty; FK cover_photo_id -> photos ON DELETE SET NULL
- add photos_count / members_count to album responses; cover repo test

Frontend:
- split layouts into (app) shell (resizable rail + contextual panel) and (public)
- album rail + create-album dialog; album panel with cover picker / make-cover
- photo viewer route with filmstrip, replacing lightbox; share reuses it
- grid density slider, toolbar + drag-and-drop upload, dark graphite default

OpenSpec: add lightroom-workspace-ui change (proposal, design, specs, tasks)

backend/migrations/000002_album_cover_fk.down.sql

@@ -0,0 +1 @@
+ALTER TABLE albums DROP CONSTRAINT IF EXISTS albums_cover_photo_fk;

backend/migrations/000002_album_cover_fk.up.sql

@@ -0,0 +1,9 @@
+-- Clear any dangling cover references before adding the constraint.
+UPDATE albums
+SET cover_photo_id = NULL
+WHERE cover_photo_id IS NOT NULL
+  AND cover_photo_id NOT IN (SELECT id FROM photos);
+
+ALTER TABLE albums
+    ADD CONSTRAINT albums_cover_photo_fk
+    FOREIGN KEY (cover_photo_id) REFERENCES photos(id) ON DELETE SET NULL;