feat(connections): browse available databases when creating/editing a connection

[exzvor]

Summary

When a user mistypes the database name, the connection to the server succeeds (host/port/auth are fine) and PG only rejects the unknown database — but the form gave no way to see which databases the credentials can actually reach (#24).

Adds a list_databases command (+ a _for_edit variant that fills the password from the keychain, mirroring test_connection_for_edit) that reuses the existing one-shot connect path but targets a maintenance DB (postgres, falling back to template1), so it works even when the typed database is wrong — which is the whole point. Query:

SELECT datname FROM pg_database
WHERE datistemplate = false AND has_database_privilege(datname, 'CONNECT')
ORDER BY datname

The connection form gets a Browse databases button that feeds the result into a <datalist> for type-ahead, keeping the field free-text so a not-yet-created DB name can still be typed.

Related issue

Fixes #24

Tests

• Backend integration test (testcontainers Postgres 17) — list_databases succeeds with a deliberately wrong typed database (proving it uses the maintenance DB), returns postgres, excludes template0/1, and is ordered. Runs for real on CI (ubuntu has Docker); skips gracefully without Docker. Verified passing locally against a real container.
• Frontend test — ConnectionForm.test.tsx: clicking Browse databases calls listDatabases and populates the <datalist> options (25 tests pass).
• Manual smoke — recommended: enter host/user (any database), click Browse → the field offers the real DB list.

Self-verification

• npm run typecheck / npm run lint clean
• npm run test green (ConnectionForm 25)
• cargo fmt --all -- --check clean; cargo clippy --lib no findings; cargo build --bin ide99 clean; integration test green
• DCO sign-off

Notes for reviewer

The list is an aid, not a constraint — the Database field stays free-text (you can still type a DB you're about to create). probe_databases/fetch_databases mirror run_probe/fetch_version; minor duplication of the TLS branch was preferred over refactoring the existing probe path to keep the blast radius small.