Show a one-time nudge towards in-room search when pressing Ctrl/Cmd+F on web

[hayaksi1]

Stacked on #34005 (defaults the Ctrl/Cmd+F search shortcut on for desktop). Please review
that one first — the diff here includes it and will reduce once it merges. This PR adds only the
web-side discoverability nudge on top.

Problem

On web, the in-room search shortcut (ctrlFForSearch) is intentionally off by default so the
browser's native Ctrl/Cmd+F find bar keeps working (see #34005 / #24359). The cost is
discoverability: a user who would prefer the shortcut to search the conversation has no idea the
option exists — pressing Ctrl/Cmd+F just opens the browser find bar forever.

Fix

The first time a web user presses Ctrl/Cmd+F while the shortcut is still off, show a one-time
toast explaining that Element can make that shortcut search the room, with an action to enable it.
It:

• only triggers on web (desktop already defaults the shortcut on and has no browser find bar to
  preserve, so it returns early there);
• shows at most once per browser — a new device-local ctrlFForSearchNudgeShown flag is set
  the first time, so it never nags again;
• does not swallow the keypress — the browser find bar still opens as usual; the toast is purely
  additive.

Tests

• New InRoomSearchNudgeToast-test.ts: shows once and not again, respects the already-enabled and
  desktop cases, and the enable action flips ctrlFForSearch.
• LoggedInView-test.tsx: the keydown handler invokes the nudge only in the eligible web case.

Relates to #22888
Relates to #24359

Checklist

• I have read through review guidelines and CONTRIBUTING.md.
• Tests written for new code (and old code if feasible).
• New or updated public/exported symbols have accurate TSDoc documentation.
• Linter and other CI checks pass.
• I have licensed the changes to Element by completing the Contributor License Agreement (CLA)