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/format.ts

@@ -0,0 +1,43 @@
+export function relativeTime(iso: string | null): string {
+  if (!iso) return "";
+  const then = new Date(iso).getTime();
+  const secs = Math.max(0, (Date.now() - then) / 1000);
+  const units: [number, string][] = [
+    [60, "s"],
+    [3600, "min"],
+    [86400, "h"],
+    [604800, "d"],
+    [2592000, "wk"],
+    [31536000, "mo"],
+  ];
+  if (secs < 60) return "just now";
+  for (let i = 0; i < units.length - 1; i++) {
+    const [, label] = units[i];
+    const next = units[i + 1][0];
+    if (secs < next) {
+      const v = Math.floor(secs / units[i][0]);
+      return `${v} ${label} ago`;
+    }
+  }
+  const years = Math.floor(secs / 31536000);
+  if (years >= 1) return `${years} yr ago`;
+  const months = Math.floor(secs / 2592000);
+  return `${months} mo ago`;
+}
+
+export function formatDuration(sec: number | null): string {
+  if (sec == null) return "";
+  const h = Math.floor(sec / 3600);
+  const m = Math.floor((sec % 3600) / 60);
+  const s = Math.floor(sec % 60);
+  const pad = (n: number) => String(n).padStart(2, "0");
+  return h > 0 ? `${h}:${pad(m)}:${pad(s)}` : `${m}:${pad(s)}`;
+}
+
+export function formatViews(n: number | null): string {
+  if (n == null) return "";
+  if (n >= 1e9) return `${(n / 1e9).toFixed(1).replace(/\.0$/, "")}B`;
+  if (n >= 1e6) return `${(n / 1e6).toFixed(1).replace(/\.0$/, "")}M`;
+  if (n >= 1e3) return `${(n / 1e3).toFixed(1).replace(/\.0$/, "")}K`;
+  return String(n);
+}