fix(nip46): kind:0 profile + NIP-98 auth via signer trait

[dmnyc]

Summary

Two commands were stuck on `get_keys_and_client` (which errors out for non-local signers), so remote-signer users couldn't:

• Update their kind:0 profile metadata — `publish_nostr_profile` failed with "this operation requires local keys".
• Authenticate image uploads via NIP-98 — `create_nip98_auth` failed identically, breaking image hosts that rely on it (e.g. blossom).

Both commands now route through `get_signer_and_client` and sign via `NostrSigner::sign_event` on an unsigned-event builder, matching the pattern `publish_market_comment` and the social commands already use. Local nsec users are unaffected; NIP-46 users now get the round-trip prompt to their bunker app like every other signed action.

`discovery::publish_profile` signature changes from `(keys: &Keys, ...)` → `(signer: &Arc, ...)`. The kind:0 merge step fetches the author pubkey through the signer instead of dereffing keys directly.

Out of scope

This is a surgical fix for the two commands the user just hit. The broader NIP-46 cleanup (logout button for remote signers, session detection / heartbeat / online flag) is tracked in memory and ships in a follow-up branch.

Test plan

• Local nsec user updates profile → succeeds (existing behaviour).
• NIP-46 remote-signer user updates profile → bunker app prompts → kind:0 published.
• NIP-46 user uploads an image (any flow that goes through `create_nip98_auth`) → bunker prompt → upload authenticated.
• cargo check, cargo fmt --check, cargo clippy --all-targets -- -D warnings all pass.