Add keyboard shortcuts help modal with centralized config

[ThomasK33]

Summary

• Add ? hotkey to open a modal showing all available keyboard shortcuts
• Create a centralized shortcuts configuration as the single source of truth
• Modal auto-generates from config — no manual sync needed between docs and handlers
• All keyboard handlers now use matchesKey() utility for consistency

Changes

New files:

• src/browser/lib/shortcuts.ts — Central config with types, definitions, and utilities
• src/browser/components/keyboard-shortcuts-modal.tsx — Modal component

Modified files:

• src/browser/components/app-shell.tsx — Add ? listener and render modal
• src/browser/components/command-palette.tsx — Use matchesKey() for Cmd+K/P
• src/browser/contexts/pr-review/useKeyboardNavigation.ts — Use matchesKey() for all PR review shortcuts

Test plan

• Press ? anywhere in the app (not in an input) — modal should open
• Verify all shortcuts are displayed and grouped correctly
• Press Escape or click outside to close the modal
• Verify existing shortcuts still work (j/k navigation, Cmd+K, etc.)

[vercel]

@ThomasK33 must be a member of the Coder team on Vercel to deploy.
- Click here to add @ThomasK33 to the team.
- If you initiated this build, request access.

Learn more about collaboration on Vercel and other options here.

[kylecarbs]

@ThomasK33 can you post screenshot pls

[ThomasK33]

@ThomasK33 can you post screenshot pls