Fix geohash participant counter omitting the local viewer

Summary

• #830 (text input in geohash/region channels): Reviewed MessageInput / ChatInputSection and geohash send paths. There is no readOnly/disabled TextField branch for location channels; BasicTextField stays enabled. Reporter symptoms (mic/photo working while text does not) also do not match current geohash UI (showMediaButtons hides media in location chats). I could not identify a reproducible composer bug in code—likely needs a device capture or clearer repro on current main.
• This change: When entering a geohash location channel, register the derived local Nostr identity in GeohashRepository immediately so participant counts and people lists are not stuck at 0/? until an external kind-20001 presence event arrives (matches #919 §2.1).

Test plan

• Join a building/neighborhood geohash channel with no other users on relays
• Confirm the channel row shows at least 1 participant (not ? / blank)
• Switch channels and verify count updates without waiting ~30s for presence sampling

Relates to #919. Does not close #830.

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