Adds Ctrl+Shift+A (Cmd+Shift+A on Mac) keyboard shortcut to toggle
between Ask and Build chat modes.

**Changes:**
- Created `useChatModeToggle` hook for shared logic
- Added shortcut to ChatInput and HomeChatInput components

**Usage:**
Press Ctrl+Shift+A (or Cmd+Shift+A on Mac) to quickly switch between
chat modes without using the UI.

<img width="434" height="123" alt="image"
src="https://github.com/user-attachments/assets/a8c167b6-2b54-46f5-8191-5019991fc8e5"
/>

Closes (#995)
    
<!-- This is an auto-generated description by cubic. -->
---

## Summary by cubic
Add a global hotkey to toggle between Ask and Build modes: Ctrl+Shift+A
(Cmd+Shift+A on Mac). Makes switching faster without touching the UI and
fulfills Linear #995.

- **New Features**
- Added useChatModeToggle hook to flip selectedChatMode, detect
platform, and track PostHog event chat:mode_toggle.
- Wired the hotkey into ChatInput and HomeChatInput via
useChatModeToggle.
  - Updated ChatModeSelector tooltip to show the shortcut.

<!-- End of auto-generated description by cubic. -->

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Adds Cmd/Ctrl + . shortcut to cycle chat modes via a new hook, and
updates the mode selector tooltip to show the shortcut.
> 
> - **UX/Hotkey**:
> - Introduces `useChatModeToggle` to cycle `settings.selectedChatMode`
(captures `chat:mode_toggle`), with platform detection
(`detectIsMac`/`useIsMac`) and `useShortcut` binding for `Cmd/Ctrl + .`.
> - **Integrations**:
> - Wires `useChatModeToggle()` into `components/chat/ChatInput.tsx` and
`components/chat/HomeChatInput.tsx` to enable the shortcut in chat
inputs.
> - **UI**:
> - Enhances `components/ChatModeSelector.tsx` tooltip to display the
platform-specific shortcut hint (`⌘ + .` or `Ctrl + .`).
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
d8cc3fff43b6eb3227623f1c084410f42392c0b3. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

---------
Co-authored-by: Will Chen <willchen90@gmail.com>