feat: add keyboard shortcuts and hints (#1138, #1222)

[ad4mou]

Summary

• Central keyboard-shortcut system: shared registry (lib/keyboard-shortcuts.ts), useKeyboardShortcuts hook (ignores keystrokes while typing in inputs/editors), and a platform.ts util (⌘ on macOS, Ctrl elsewhere), mounted once in _sidebar-layout.tsx.
• Shortcuts: Mod+B toggle sidebar, Mod+K command palette, Shift+Mod+O new chat, Shift+Mod+S Stories, Mod+/ keyboard-shortcuts dialog.
• Discoverability (feature: Add keyboard shortcut hints on hover #1222): sidebar rows show their shortcut inline on hover (existing style; Stories now shows ⇧⌘S), a Keyboard shortcuts help dialog, and "Keyboard shortcuts" + "Go to Stories" command-menu entries.
• Cleanup: removed the buggy Mac-only ⇧⌘O listener and the duplicate ⌘K listener; de-duplicated navigator.platform checks via the shared util.

How to test

Just do Command/Ctrl + / to see this

Also check out the keyboard shortcut hint which shows when hovering over the toggle sidebar button

Closes #1138
Closes #1222

[github-actions]

🚀 Preview Deployment

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Commit	eee05a6

⚠️ No LLM API keys configured - you'll see the API key setup flow when trying to chat.

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[cubic-dev-ai]

1 issue found across 10 files

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="apps/frontend/src/hooks/use-keyboard-shortcuts.ts">

<violation number="1" location="apps/frontend/src/hooks/use-keyboard-shortcuts.ts:21">
P2: Platform-specific shortcuts also fire with the other platform's modifier: e.g. `Ctrl+B` toggles the sidebar on macOS despite the advertised `⌘B`. Make matching choose `metaKey` on macOS and `ctrlKey` elsewhere, matching displayed shortcuts.</violation>
</file>

<sub>Reply with feedback, questions, or to request a fix.

Re-trigger cubic</sub>

[cubic-dev-ai]

1 issue found across 6 files (changes from recent commits).

Prompt for AI agents (unresolved issues)

Check if these issues are valid — if so, understand the root cause of each and fix them. If appropriate, use sub-agents to investigate and fix each issue separately.


<file name="apps/frontend/src/lib/stories-cache.ts">

<violation number="1" location="apps/frontend/src/lib/stories-cache.ts:12">
P2: Opening Stories through this helper can retain a stale archived shared-stories list for up to the query stale time. Include `story.listSharedArchived` with the other story cache keys.</violation>
</file>

<sub>Tip: Review your code locally with the cubic CLI to iterate faster.

Re-trigger cubic</sub>