perf(watch): a shared link loads the player, not the app

A /watch link is opened by whoever it was sent to — signed out, often on a phone on mobile data —
and it booted through main.tsx: every app provider, the app's i18n (every locale of every area),
the flag sprites, ~392 KB gzip before the player could start. It now has its own HTML entry,
watch.html → watchMain.tsx, which imports WatchPage and what it renders; the server hands every
/watch/ path that page (the OG block injected as before), and vite dev does the same.

Measured on the build: /watch JS 290 → 87 KB gzip, CSS 104 → 19 KB; the app's own bytes unchanged.
Two leaks had to go for that: lib/format imported the app's i18n for one "just now" string (the
relative-time formatters now take `t`, like the file's other t-formatters), and the boundary's
crash copy came from the same i18n (CrashBoundary takes its copy; ErrorBoundary is the app's
wrapper, the watch entry gives it the page's own strings). The page's popover tree went too: it
renders no popover.

Held by: a contract test walking the entry's runtime imports, naming the path to anything only the
app may load (red on a planted i18n import in lib/format), and holding watch.html's head equal to
index.html's; a DB-lane test that every /watch/ path gets the player's page with the link's OG tags
and app paths get the app (red on the old routing). Screenshots at 375 px, EN and HU, video / audio /
password / gone / wrong password / unlocked: identical to the old entry's, bar the video's buffered bar.

frontend/src/watchMain.tsx

@@ -0,0 +1,26 @@
+import React from "react";
+import { createRoot } from "react-dom/client";
+import { CrashBoundary } from "./components/CrashBoundary";
+import WatchPage from "./components/WatchPage";
+import { T } from "./components/watchStrings";
+import "./index.css";
+import { installStaleBuildReload } from "./lib/staleBuild";
+
+// The entry of a public /watch/<token> link (watch.html). Whoever opens it got the link from a
+// friend: signed out, often on a phone, often on mobile data — so it imports the player page and
+// what that page renders, and none of main.tsx's app (no providers, no app i18n, no client-log
+// wiring: an anonymous viewer has no diagnostics consent to send it under). A contract test walks
+// this file's imports and fails on any of those (`watchEntry.contract.test.ts`).
+
+// A tab left open across a release reloads onto the new build instead of failing on a chunk.
+installStaleBuildReload();
+
+const crashCopy = () => ({ title: T.crashTitle, subtitle: T.crashHint, reload: T.reload });
+
+createRoot(document.getElementById("root")!).render(
+  <React.StrictMode>
+    <CrashBoundary copy={crashCopy}>
+      <WatchPage />
+    </CrashBoundary>
+  </React.StrictMode>,
+);