Settings persistence and multi-repo management

[jimmytacks]

Part of #1

Description

Persist settings and allow managing multiple GitHub repo sources.

Deliverables

• Settings.cs — JSON persistence to %APPDATA%\LambdaBoss\settings.json. Properties: Repos (list of RepoConfig), KeyboardShortcut (default ^+L), CacheTtlMinutes
• LambdaPopup.xaml — Add settings section for managing repos (add URL, toggle enable/disable, remove)
• RibbonController.cs — Lambda Boss ribbon tab: Load Library button, Settings, About, Update notification

Depends on

• Full popup UI: search, browse, and prefix prompt #5 (full popup UI)