Full popup UI: search, browse, and prefix prompt

[jimmytacks]

Part of #1

Description

Replace the minimal tracer-bullet popup with the full search/browse experience.

Deliverables

• LambdaPopup.xaml — Two modes:
  1. Library mode (default): list available libraries grouped by repo, select → prefix prompt → load all
  2. Search mode: type to filter across all LAMBDAs, select → loads containing library
• LambdaPopup.xaml.cs — Keyboard-first UX:
  • Type to search (auto-switch to search mode)
  • Up/Down to navigate, Enter to load, Tab to switch modes, Escape to close
  • Inline prefix prompt pre-filled with default_prefix
• Window positioning via WindowPositioner.CenterOnExcel from Taglo.Excel.Common

Depends on

• GitHub fetching and caching #4 (GitHub fetching)