fix(recording): honor the selected save folder

[themaddoxed]

Description

Adds a native Choose Folder action next to Open Folder and makes the persisted recording root apply to all future live recordings and future audio imports.

Behavior

• Existing meetings keep their stored folder_path and are not moved.
• New live recordings capture the selected root when recording starts.
• New audio imports capture the selected root before the background import starts.
• Invalid or unavailable configured roots fail before recording/import output begins; there is no silent fallback to the default folder.
• Folder changes are validated and persisted transactionally, with rollback and serialized reads/writes.
• The folder-only command reloads the latest preferences under the lock, so unrelated device and auto-save settings are preserved.
• The UI changes only after persistence succeeds, blocks duplicate/racing controls, and reports picker/save/open errors.
• Analytics records only folder_selected: true; no filesystem path is sent.

Related Issue

Fixes #481

Related to #516. This PR covers the same live-recording and import paths while adding transactional persistence, fail-fast destination handling, race protection, privacy-safe analytics, and focused tests.

Type of Change

• Bug fix
• New feature
• Documentation update
• Performance improvement
• Code refactoring
• Other

Testing

• Unit tests added/updated
• Manual testing performed
• All feature tests pass

Clean upstream/devtest branch:

• 6 recording-preference persistence/concurrency tests passed.
• 2 live-recording root tests passed.
• 3 import-root/snapshot tests passed.
• pnpm run build passed.

Source build with browser-test infrastructure:

• Full Rust suite: 204 passed, 0 failed, 3 ignored.
• Playwright: 14 passed (folder contract plus settings/theme regression).
• pnpm exec tsc --noEmit passed.
• pnpm run build passed.

Known upstream baseline checks outside this diff:

• The full devtest Rust suite has one exact-duration float assertion failure: 159.999996ms != 160ms in audio::device_detection::tests::test_calculate_buffer_timeout_bluetooth. This PR does not modify that file.
• Direct project-wide tsc --noEmit on devtest reports the existing missing bun:test declaration in tests/lib/blocknote-markdown.test.ts; the production Next.js build passes type checking.

Documentation

• Documentation updated
• No documentation needed

Checklist

• Code follows project style
• Self-reviewed the code
• Added comments/tests for non-obvious concurrency behavior
• README update is not needed