feat(theming): in-UI theme editor (2.10.0)

Recolour the whole UI from Settings — 12 palette colour pickers with
live preview, presets (Ember/Frost/Verdant/Amethyst), and reset to the
default Blackflame. Overrides persist in app_settings (key 'theme') via
a hex-validated /api/theme route and apply to :root on boot.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

public/style.css

@@ -699,6 +699,12 @@ body.drawer-open #scrim { opacity: 1; pointer-events: auto; }
 .st-table td { padding: 5px 10px; border-bottom: 1px solid #ffffff08; }
 .st-table td.num { text-align: right; }
 
+/* ---- Theming panel ---- */
+.theme-grid { display: grid; grid-template-columns: repeat(auto-fill, minmax(200px, 1fr)); gap: 8px 18px; margin-bottom: 14px; }
+.theme-row { display: flex; align-items: center; justify-content: space-between; gap: 10px; font-size: 12px; color: var(--muted); }
+.theme-row input[type=color] { width: 40px; height: 24px; padding: 0; border: 1px solid var(--border); border-radius: 4px; background: none; cursor: pointer; }
+.theme-actions { display: flex; align-items: center; gap: 10px; flex-wrap: wrap; }
+
 .hidden { display: none !important; }
 
 /* Storage card (sv-cluster container) — warn dot + capacity meter + subheader */