docs: add UI-first design principle

Every player-triggered action (new game, undo, draw, pause, open any
overlay, switch mode, etc.) must be reachable from a visible UI
control. Keyboard shortcuts are optional accelerators only — never
the sole entry point. New gameplay features ship with the UI control
alongside the system that backs it.

- ARCHITECTURE.md §1 (Design Principles): add UI-first bullet.
- ARCHITECTURE.md §5 plugin table: rename "Key" column to
  "Shortcut" and add a note that the column lists optional
  accelerators, not primary entry points.
- CLAUDE.md (Bevy Conventions): add a matching hard rule.

Surfaced during smoke testing: the N+N "press again to confirm"
toast collides with the ConfirmNewGameScreen modal because the
keyboard flow is the only entry point. Adding a visible New Game
button (next commit) makes the modal the single source of truth for
the confirm flow.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

CLAUDE.md

@@ -77,6 +77,7 @@ cargo clippy -p solitaire_core -- -D warnings
 - Resources own shared state. Events communicate between systems. Components own per-entity data.
 - All UI screens are built with Bevy UI (`bevy::ui`). Never mix UI layout and game logic in the same system.
 - Layout is recomputed on `WindowResized` — never assume a fixed window size.
+- **UI-first.** Every player-triggered action (new game, undo, draw, pause, open stats / settings / help / profile / leaderboard, switch mode, etc.) must be reachable from a visible UI control. Keyboard shortcuts are optional accelerators — never the sole entry point. New gameplay features ship with the UI control alongside the system that backs it; do not merge a feature that is keyboard-only.
 
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