feat: populate contacts from message senders automatically

[Edouard-m-333]

Problem

contacts is only populated from the address book (dialog peers and contacts.* lookups). The sender of a message from someone not in your address book — common in groups/supergroups — has no stored name, so message output and SQL analytics fall back to user:<id>. The name can't be recovered later from a bare numeric id, because resolving a user requires its access_hash.

Change

Every fetched message and every real-time update already carries its sender as a Peer, but only the bare id was retained. This upserts the sender into contacts at each point a message is processed — no extra API calls, no schema change, no new dependencies:

• daemon (src/cmd/daemon.rs): each incoming NewMessage upserts its sender → contacts stay current in real time.
• sync (src/app/sync.rs): the three message paths (sync, sync_msgs, deep fetch) upsert the senders of messages they pull. Adds a SenderContact field on FetchedMessage and a small sender_contact_from_peer helper.
• chats members (src/cmd/chats.rs): persists every participant it lists (also covers members who haven't posted).

All four reuse the existing idempotent Store::upsert_contact, whose ON CONFLICT … DO UPDATE uses COALESCE/non-empty guards, so a good value is never overwritten with an empty one. Failures are best-effort (logged in the daemon, ignored elsewhere) so contact population never breaks message storage.

Testing

• cargo build --release — clean.
• Ran the daemon, sync / sync msgs, and chats members against a live account: group senders that previously showed as user:<id> now resolve to names in contacts; re-runs are idempotent and don't clobber existing names.