Store: one tab combining books, free audiobooks and radio, plus My Store for your own books, music, apps and sandboxed extensions

- Store tab with sections; the book store, LibriVox and radio finder are shown inside it (old entry points redirect)
- My Store: admin uploads files or copies from Seafile; books go to a person's shelf, music plays in the player, apps download
- Extensions: one HTML file run in a sandboxed frame with a small storage/toast/user bridge; example in docs/extensions

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

public/style.css

@@ -961,3 +961,7 @@ body.vizopen { overflow: hidden; }
 .ctsug { position: absolute; z-index: 30; left: 0; right: 0; background: var(--panel); border: 1px solid var(--line); border-radius: 10px; box-shadow: 0 6px 20px rgba(0,0,0,.15); max-height: 220px; overflow: auto; }
 .ctsug button { display: block; width: 100%; text-align: left; padding: 8px 12px; border: 0; background: none; color: inherit; font: inherit; cursor: pointer; } .ctsug button:hover, .ctsug button.on { background: rgba(127,127,127,.15); }
 .pdhit { margin-bottom: 8px; } mark { background: rgba(255, 200, 0, .45); color: inherit; border-radius: 3px; }
+
+/* Store */
+#stbody .wrap { padding: 0; max-width: none; }
+#stbody .vimg img { width: 100%; height: 100%; object-fit: cover; }