feat: M4 (part 2) — React reader UI with theming

- Vite + React + TS + Tailwind SPA served by FastAPI (multi-stage Docker build)
- Four color schemes (Midnight default, Forest, Slate, YouTube) x dark/light,
  adjustable text size; persisted to user preferences and localStorage
- Header search, grid/list toggle, theme menu; sidebar filters (show state,
  sort, include Shorts/live, language + topic tag chips with any/all matching)
- Infinite-scroll feed of video cards; click opens youtube.com in a new tab;
  per-video watched / saved / hidden actions with optimistic updates
- SPA fallback routing; login screen for unauthenticated users

frontend/src/lib/theme.ts

@@ -0,0 +1,42 @@
+export type Mode = "dark" | "light";
+export type Scheme = "midnight" | "forest" | "slate" | "youtube";
+
+export const SCHEMES: { id: Scheme; name: string; swatch: string }[] = [
+  { id: "midnight", name: "Midnight", swatch: "#6d8cff" },
+  { id: "forest", name: "Forest", swatch: "#2dd4bf" },
+  { id: "slate", name: "Slate", swatch: "#e8913c" },
+  { id: "youtube", name: "YouTube", swatch: "#ff3b46" },
+];
+
+export interface ThemePrefs {
+  mode: Mode;
+  scheme: Scheme;
+  fontScale: number;
+}
+
+export const DEFAULT_THEME: ThemePrefs = {
+  mode: "dark",
+  scheme: "midnight",
+  fontScale: 1.06,
+};
+
+export function applyTheme(t: ThemePrefs): void {
+  const el = document.documentElement;
+  el.dataset.theme = t.mode;
+  el.dataset.scheme = t.scheme;
+  el.style.setProperty("--font-scale", String(t.fontScale));
+}
+
+const KEY = "subfeed.theme";
+
+export function loadLocalTheme(): ThemePrefs {
+  try {
+    return { ...DEFAULT_THEME, ...JSON.parse(localStorage.getItem(KEY) || "{}") };
+  } catch {
+    return DEFAULT_THEME;
+  }
+}
+
+export function saveLocalTheme(t: ThemePrefs): void {
+  localStorage.setItem(KEY, JSON.stringify(t));
+}