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/components/ui/RelativeTime.dom.test.tsx

@@ -4,7 +4,7 @@
 import { afterEach, describe, expect, it } from "vitest";
 import { cleanup, render } from "@testing-library/react";
 import { formatDateTime, formatDateTimeSec, relativeTime } from "../../lib/format";
-import "../../i18n"; // the real instance, in English: the component names time in the UI language
+import i18n from "../../i18n"; // the real instance, in English: the component names time in the UI language
 import { RelativeTime } from "./RelativeTime";
 
 afterEach(cleanup);
@@ -15,7 +15,7 @@ describe("RelativeTime", () => {
   it("says how long ago, and names the exact minute on hover", () => {
     const { container } = render(<RelativeTime at={THREE_DAYS_AGO} />);
     const el = container.querySelector("time")!;
-    expect(el.textContent).toBe(relativeTime(THREE_DAYS_AGO, "en"));
+    expect(el.textContent).toBe(relativeTime(THREE_DAYS_AGO, "en", i18n.t));
     expect(el.getAttribute("title")).toBe(formatDateTime(THREE_DAYS_AGO, "en"));
     expect(el.getAttribute("dateTime")).toBe(THREE_DAYS_AGO);
   });