feat: band NC folder config, fix watcher event filter, add light/dark theme

- Add PATCH /bands/{id} endpoint for admins to update nc_folder_path
- Add band NC scan folder UI panel with inline edit
- Fix watcher: use activity type field (not human-readable subject) for upload detection
- Reorder watcher filters: audio extension check first, then band path, then type
- Add dark/light theme toggle using GitHub Primer-inspired CSS custom properties
- All inline styles migrated to CSS variables for theme-awareness

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

web/src/index.css

@@ -0,0 +1,52 @@
+*, *::before, *::after {
+  box-sizing: border-box;
+}
+
+body {
+  margin: 0;
+  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", system-ui, sans-serif;
+  -webkit-font-smoothing: antialiased;
+  background: var(--bg);
+  color: var(--text);
+}
+
+input, textarea, button, select {
+  font-family: inherit;
+}
+
+/* ── Dark theme (default) — GitHub Primer slate ─────────────────────────── */
+:root,
+[data-theme="dark"] {
+  --bg:          #0d1117;
+  --bg-subtle:   #161b22;
+  --bg-inset:    #21262d;
+  --border:      #30363d;
+  --text:        #e6edf3;
+  --text-muted:  #8b949e;
+  --text-subtle: #484f58;
+  --accent:      #f0a840;
+  --accent-bg:   #2a1e08;
+  --accent-fg:   #080c10;
+  --teal:        #38c9a8;
+  --teal-bg:     #0a2820;
+  --danger:      #f85149;
+  --danger-bg:   #1a0810;
+}
+
+/* ── Light theme ─────────────────────────────────────────────────────────── */
+[data-theme="light"] {
+  --bg:          #ffffff;
+  --bg-subtle:   #f6f8fa;
+  --bg-inset:    #eef1f5;
+  --border:      #d0d7de;
+  --text:        #24292f;
+  --text-muted:  #57606a;
+  --text-subtle: #afb8c1;
+  --accent:      #f0a840;
+  --accent-bg:   #fff8e1;
+  --accent-fg:   #1c1007;
+  --teal:        #0a6b56;
+  --teal-bg:     #d1f7ef;
+  --danger:      #cf222e;
+  --danger-bg:   #ffebe9;
+}