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)

frontend/src/app.html

@@ -1,5 +1,5 @@
 <!doctype html>
-<html lang="ru" data-theme="light">
+<html lang="ru" data-theme="dark">
 	<head>
 		<meta charset="utf-8" />
 		<meta name="viewport" content="width=device-width, initial-scale=1" />
@@ -7,7 +7,7 @@
 		<script>
 			(function () {
 				try {
-					var stored = localStorage.getItem('theme') || 'system';
+					var stored = localStorage.getItem('theme') || 'dark';
 					var resolved = stored;
 					if (stored === 'system') {
 						resolved = window.matchMedia('(prefers-color-scheme: dark)').matches