feat: merge pipeline, per-user credentials, unified picker, group contacts

- Enable merge UI for linking channel contacts to tenant_users
- Contact → tenant_user resolution with cached lookup (60s TTL)
- MCP per-user credentials via user-keyed connection pool
- Secure CLI per-user credentials with AES-256-GCM encryption
- Unified UserPickerCombobox searching contacts + tenant_users
- Group contact collection with chat title in all channels
- Group permission inheritance via wildcard user_id="*"
- Fix heartbeat using wrong userID in group chats
- Filter internal senders from contact collection
- Add contact_type column (user/group) to channel_contacts
- SQLite schema v2 migration for desktop edition

internal/store/sqlitestore/schema.sql

@@ -1036,6 +1036,7 @@ CREATE TABLE IF NOT EXISTS channel_contacts (
     username         VARCHAR(255),
     avatar_url       TEXT,
     peer_kind        VARCHAR(20),
+    contact_type     VARCHAR(20) NOT NULL DEFAULT 'user',
     metadata         TEXT DEFAULT '{}',
     merged_id        TEXT,
     tenant_id        TEXT NOT NULL REFERENCES tenants(id),