feat(ui): accordion sidebar panels + wood texture preview swatches

Sidebar appearance menu improvements:

Accordion behavior:
- Lift panel open/close state to a shared 'openPanel' variable on the
  parent container so only one sidebar panel (User, Appearance, Admin,
  Sign In) can be open at a time; all can be closed.
- Admin panel still auto-opens on /admin/* pages via initial state.
- Add chevron rotation to User and Appearance panels (previously only
  Admin rotated); add a chevron to the Sign In panel for consistency.

Wood texture previews:
- Generate 48x48 WebP thumbnails (~200 bytes each) from the full-size
  PNG textures (873 KB – 1.9 MB) so the bookshelf option circles show
  the actual wood grain instead of a flat grey dot.
- Use unquoted url() in the inline style to avoid templ's double-HTML
  escaping of single quotes (SanitizeStyleAttributeValues + EscapeString
  turned url('...') into url('...) which is invalid CSS).
- 'None' keeps the flat neutral circle.

CSS resilience:
- Move the wood background-image: url() rules from the compiled
  style.css into input.css (the Tailwind source) so they survive CSS
  rebuilds instead of being silently lost.

web/static/input.css

@@ -649,6 +649,15 @@
     background-position: center;
     background-size: cover;
   }
+  .bg-wood-dark {
+    background-image: url(/static/textures/wood-dark.png);
+  }
+  .bg-wood-light {
+    background-image: url(/static/textures/wood-light.png);
+  }
+  .bg-wood-mahogany {
+    background-image: url(/static/textures/wood-mahogany.png);
+  }
 
   body.bg-wood-dark,
   body.bg-wood-light,