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.

backend/app/downloads/og.py

@@ -147,13 +147,14 @@ def _watch_tags(token: str, db: Session) -> str | None:
     return "\n    ".join(tags)
 
 
-def render_watch_html(token: str, index_html: Path, db: Session) -> str | None:
-    """Return index.html with the OG block replaced for this watch token, or None to serve the
-    generic page unchanged (invalid/expired/not-ready link, or a template without the markers)."""
+def render_watch_html(token: str, page_html: Path, db: Session) -> str | None:
+    """Return the watch page (watch.html) with the OG block replaced for this token, or None to
+    serve the generic page unchanged (invalid/expired/not-ready link, or a template without the
+    markers)."""
     tags = _watch_tags(token, db)
     if tags is None:
         return None
-    text = index_html.read_text(encoding="utf-8")
+    text = page_html.read_text(encoding="utf-8")
     if "<!--OG:START-->" not in text:
         return None
     # A function replacement avoids re.sub interpreting backslashes/group refs in `tags`.