Warn when an encrypted search runs before the index has finished building

[hayaksi1]

Problem

When a search runs in an encrypted room while the local Seshat index is still being
built (crawling not-yet-indexed history), the search can silently return partial
results, with nothing to tell the user that more matches may appear once indexing
finishes (#32253).

Fix

SearchWarning now observes the event index's crawl progress:

• A new useIsCrawlInProgress hook subscribes to the index's changedCheckpoint event
  (emitted as each checkpoint drains) and tracks whether currentRoom() is still
  non-null — i.e. the crawl is still running.
• While a crawl is in progress and the warning is being shown for a search
  (WarningKind.Search), a polite live-region notice — "Results may be incomplete
  because your search index is still being built." — is rendered with role="status",
  so a screen reader announces it if it appears while a search panel is already open.
• The notice clears automatically the moment indexing finishes (the hook re-evaluates on
  every checkpoint change), and the listener is removed on unmount.

No behaviour changes for unencrypted rooms, for the no-index / index-error paths, or for
the files warning.

Tests

SearchWarning-test.tsx covers: no warning when the index is complete, the partial-results
warning while crawling, auto-clear when the crawl finishes, listener cleanup, and that the
notice is scoped to search rather than the files warning.

Fixes #32253

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)

[t3chguy]

Please include a screenshot of the warning for design to review