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/theme.ts

@@ -0,0 +1,30 @@
+import { createContext, useContext, useState, useEffect, type ReactNode } from "react";
+import { createElement } from "react";
+
+type Theme = "dark" | "light";
+
+interface ThemeCtx {
+  theme: Theme;
+  toggle: () => void;
+}
+
+const ThemeContext = createContext<ThemeCtx>({ theme: "dark", toggle: () => {} });
+
+export function ThemeProvider({ children }: { children: ReactNode }) {
+  const [theme, setTheme] = useState<Theme>(() => {
+    return (localStorage.getItem("rh_theme") as Theme) ?? "dark";
+  });
+
+  useEffect(() => {
+    document.documentElement.dataset.theme = theme;
+    localStorage.setItem("rh_theme", theme);
+  }, [theme]);
+
+  const toggle = () => setTheme((t) => (t === "dark" ? "light" : "dark"));
+
+  return createElement(ThemeContext.Provider, { value: { theme, toggle } }, children);
+}
+
+export function useTheme(): ThemeCtx {
+  return useContext(ThemeContext);
+}