feat: fetch databases in connection dialog

[jonasnobile]

Summary

Adds a Fetch button next to the Database field in the connection dialog that connects with the entered credentials, queries the server's catalog, and replaces the input with a dropdown to pick from. Saves users from having to know or type the database name when adding a new connection. Works for PostgreSQL and MySQL.

How it works

• Backend: new ConnectionManager.listDatabasesForConfig(config) creates an ephemeral driver (same pattern as testConnection), connects, runs a dialect-specific catalog query (pg_database / information_schema.schemata), then disconnects. SSH tunnels are set up and torn down too. When the user hasn't picked a database yet, falls back to postgres (PG) or information_schema (MySQL). Exposed as the databases.listForConfig RPC.
• Frontend: ConnectionDialog owns the fetched-list state. The list is auto-cleared whenever the user changes a field that targets a different server — type, host, port, user, password, SSL mode, SSH tunnel, or a pasted URL. Prevents the case where a user fetches against server A, picks a database, edits the host to server B, and silently saves a config that mixes the two.

Why "ephemeral" matters

The existing databases.list RPC requires a saved + connected connectionId, which doesn't exist while the user is filling out the dialog. The new method follows the same ephemeral pattern as testConnection so no state is registered in the connection manager — verified by a test (does not leak active drivers).

Test plan

• bun test tests/list-databases-for-config.test.ts — 7 integration tests against PG + MariaDB (docker-compose):
  • returns seeded DB, excludes template0/template1
  • returns seeded DB, excludes mysql/information_schema/performance_schema/sys
  • sorted alphabetically
  • falls back to postgres / information_schema when DB is empty
  • bad credentials reject
  • no driver leak (ephemeral cleanup verified)
• bun test tests/connection-manager.test.ts — added SQLite-rejection test
• Manual UI verification still required — open the dialog, click Fetch, change host, verify the dropdown reverts to text input. I was unable to drive the browser from this session.

CI status

This branch is the feature commit only. CI's format:check, lint, and tsc will fail on pre-existing-on-main issues (CLAUDE.md trailing newline, two biome warnings in indexeddb.ts/grid.ts, a literal-key access in query-executor.ts, an unused isLinux in backend-desktop/index.ts). Those are addressed by the companion PR #7 — merge it first, then rebase this one and CI will go green. The single line in tests/connection-manager.test.ts that overlaps between the two PRs is identical (both remove the same trailing blank line), so merge order doesn't matter for that file.

Known follow-ups (not in this PR)

• PG postgres fallback can fail on hardened installs that drop the default DB; error message will be opaque rather than actionable.
• MySQL validateConfig requires non-empty password (pre-existing — same as testConnection); local trust auth would surface here too.
• The Fetch RPC sends the cleartext password the same way connections.test does — same surface as before, but worth noting that the dialog now has two paths that transmit creds.

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