feat: add hotkey to toggle chat modes (#1274) (#995) (#1284)

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>

src/components/ChatModeSelector.tsx

@@ -13,6 +13,7 @@ import {
 import { useSettings } from "@/hooks/useSettings";
 import type { ChatMode } from "@/lib/schemas";
 import { cn } from "@/lib/utils";
+import { detectIsMac } from "@/hooks/useChatModeToggle";
 
 export function ChatModeSelector() {
   const { settings, updateSettings } = useSettings();
@@ -35,6 +36,7 @@ export function ChatModeSelector() {
         return "Build";
     }
   };
+  const isMac = detectIsMac();
 
   return (
     <Select value={selectedMode} onValueChange={handleModeChange}>
@@ -53,7 +55,14 @@ export function ChatModeSelector() {
             <SelectValue>{getModeDisplayName(selectedMode)}</SelectValue>
           </MiniSelectTrigger>
         </TooltipTrigger>
-        <TooltipContent>Open mode menu</TooltipContent>
+        <TooltipContent>
+          <div className="flex flex-col">
+            <span>Open mode menu</span>
+            <span className="text-xs text-gray-200 dark:text-gray-500">
+              {isMac ? "⌘ + ." : "Ctrl + ."} to toggle
+            </span>
+          </div>
+        </TooltipContent>
       </Tooltip>
       <SelectContent align="start" onCloseAutoFocus={(e) => e.preventDefault()}>
         <SelectItem value="build">