feat(backup): add File > Backup… for PostgreSQL via pg_dump

[samirmhsnv]

Summary

• New File > Backup… menu item for PostgreSQL and Redshift connections. Pick a database, choose a destination, runs pg_dump -Fc with live byte progress and cancel.
• Reuses the connection's active SSH tunnel via session.effectiveConnection, so tunneled connections work without opening a second port forward.
• pg_dump is auto-detected from PATH and common Homebrew locations. Override under Settings > Terminal > CLI Paths > pg_dump.

Notes for reviewers

• DatabaseSwitcherSheet gains a Mode enum (.switch default, .backup). Existing call sites stay on the default and are unchanged. In backup mode the schemas tab and create/drop affordances are hidden, the primary button reads "Back Up…", and the callback does not auto-dismiss so the host sheet can transition to the save panel.
• PostgresBackupService polls file size every 250ms for progress (pg_dump has no progress hook). Cancel sends SIGTERM and removes the partial file. Stderr is captured (capped at 64KB) and shown on failure.
• SSL mode raw values are mapped to libpq's PGSSLMODE values (require, verify-ca, verify-full, etc.) instead of being passed through verbatim.
• --no-password is passed to pg_dump so it fails fast with an auth error instead of hanging on a TTY prompt when PGPASSWORD is missing.
• Headless xcodebuild from CLI fails locally on a SwiftLint plugin sandbox issue in CodeEditTextView/CodeEditSourceEditor (unrelated to this change). SwiftLint --strict is clean on every changed file. Verify the build from Xcode.

Test plan

• Postgres connection without SSH: open Backup…, pick a DB, save, verify the .dump file is a valid custom-format archive (pg_restore -l file.dump).
• Postgres connection with SSH tunnel: same flow, verify it uses the existing tunnel (no second port forward in lsof).
• Cancel mid-backup: confirm the partial file is removed and the sheet shows "Backup Cancelled".
• Non-Postgres connection: Backup… menu item is disabled.
• Custom pg_dump path: set under Settings > Terminal > CLI Paths > pg_dump, verify it is used.
• No pg_dump on system: backup fails with the install hint.
• Redshift connection: backup works (Redshift shares the PostgreSQL plugin).

[github-actions]

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1 out of 2 committers have signed the CLA.
✅ (samirmhsnv)[https://github.com/samirmhsnv]
❌ @samir Mammadhasanov
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[samirmhsnv]

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[samirmhsnv]

Superseded by #1211, which covers both Backup and Restore with a combined description and test plan.