Implement TUI professor ratings screen and keybindings

[kleinpanic]

Parent: #43

Add a dedicated TUI surface for professor ratings.

Goals

• create a new screen for professor/rating data
• add discoverable keybinding(s)
• keep visual parity with existing DataTable/detail-pane screens
• support per-course instructor context and empty/error states

Deliverables

• new screen
• app wiring and keybindings
• help text updates
• visual polish

[kleinpanic]

RMP TUI screen (src/canvas_tui/screens/rmp.py) ships in Phase 8 plan 08-02. Three-state screen: search → results → details, consuming existing rmp/ data layer. Accessible via R keybinding from HomeScreen.