fix: address 11 bugs found in codebase review

[oth-body]

1. Fix eventCache shadowing with := in main() — use plain =
2. Fix getDMs() nip04.Decrypt using raw privateKey instead of computed shared secret
3. Fix text input not updated for cursor states 51 and 200 in TUI login
4. Fix NIP-89 a tag format from pubkey:d:kind to kind:pubkey:d
5. Fix NIP-89 handler registration kind 1984 → 31990 with d tag
6. Add bounds check len(tag) > 1 before accessing tag[1] in findHandlers
7. Capture withLoading errors for DMs and Replies instead of discarding
8. Fix context leak: call cancel() immediately instead of defer in relay loop
9. Add upper bound clamp (max 7) for Home screen cursor
10. Fix publishToAllRelays to only error when zero relays succeed
11. Check GetPublicKey and Sign errors in publishPostTUI