Mentions: show typeahead suggestions as the writer types
The @mention autocompleter reused lookupActor, an exact
app.bsky.actor.getProfile resolve gated by isValidHandleOrDid. The
suggestion menu therefore only appeared once a writer typed a complete,
valid handle; partial input ('ria') failed validation and surfaced
nothing, making the feature effectively undiscoverable.
Add searchActors, which queries the public, unauthenticated
app.bsky.actor.searchActorsTypeahead and returns a short list of
matches, and point the completer's options() at it. Matches now appear
as the writer types, mirroring the Bluesky composer.
searchActors keeps lookupActor's safety posture (fixed host constant, q
URL-encoded as the only user input, never throws -- resolves to [] on
any failure) but is deliberately not gated by isValidHandleOrDid, since
partial input is the whole point. The inserted skypress/mention anchor
and pick-time DID resolution are unchanged.