Expose Claude keychain prompt policy in Preferences

[ratulsarna]

Summary

• Expose Claude OAuth keychain prompt policy in Preferences → Providers → Claude with options: Never, Only on user action, Always allow prompts.
• Keep the control always visible; disable it (with explanatory subtitle) when Advanced → Disable Keychain access is enabled.
• Wire claudeOAuthKeychainPromptMode persistence/defaulting through SettingsStore using the claudeOAuthKeychainPromptMode defaults key (fallback: onlyOnUserAction).
• Add picker isEnabled support in shared settings descriptors and row rendering.
• Add/extend tests for descriptor coverage, persistence/defaulting, and pane smoke behavior.
• Update docs (docs/claude.md, docs/ui.md) and changelog.

Validation

• swift test
• pnpm check
• ./Scripts/compile_and_run.sh

All commands passed locally.