feat(desktop): add right-click context menu to rename sessions in sidebar

[tfsq]

Summary

Adds right-click context menu to session items in the sidebar for quick renaming.

Changes

• Add onContextMenu handler to session items in AppSidebar.tsx
• Add RenameModal component for inline editing
• Wire to existing updateSessionName API
• Dispatch SESSION_RENAMED event to sync with other components
• Toast notification on success/failure

Motivation

Current rename flow requires: Sidebar → View All → hover → pencil icon. This is buried. Right-click rename is standard UX in chat apps (Slack, Discord, iMessage).

Closes #6754

[zanesq]

@tfsq really appreciate it! I think we can reuse our existing code for the rename and save on some of the added code here was some feedback from goose, do you mind taking a look?

## PR #6841 Analysis: Right-Click Context Menu for Session Rename

### Summary
- **+190 lines / -3 lines** in `AppSidebar.tsx`
- Adds right-click context menu to rename sessions in the sidebar

### Issues with the Current Approach

The PR introduces a lot of **duplicated code** that already exists elsewhere in the codebase:

1. **`RenameModal` is nearly identical to `EditSessionModal`** in `SessionListView.tsx` (lines 52-130)
   - Same structure, same logic, same styling
   - Only difference is the title text ("Rename Session" vs "Edit Session Description")

2. **Custom context menu implementation** instead of using existing Radix UI components
   - The codebase already has `DropdownMenu` components (`ui/dropdown-menu.tsx`) built on Radix UI
   - Radix also provides `@radix-ui/react-context-menu` (available via the `radix-ui` package in node_modules)
   - The PR manually handles click-outside, escape key, positioning - all of which Radix handles automatically

3. **Duplicate event handling logic** for `SESSION_RENAMED` event dispatch

### Recommendations to Simplify

Here's how to reduce this PR from ~190 lines to ~40-50 lines:

#### Option 1: Reuse Existing Modal (Recommended)

1. Extract EditSessionModal from SessionListView.tsx to a shared component
   e.g., ui/desktop/src/components/ui/EditSessionModal.tsx

2. In AppSidebar.tsx, just import and use it:
   import { EditSessionModal } from '../ui/EditSessionModal';

3. Use DropdownMenu with onContextMenu trigger instead of custom context menu

#### Option 2: Use Radix ContextMenu

Use the existing Radix primitives - create a simple context-menu.tsx wrapper
similar to how dropdown-menu.tsx wraps @radix-ui/react-dropdown-menu

import * as ContextMenuPrimitive from '@radix-ui/react-context-menu';
// ... wrap with consistent styling

### Specific Code to Remove/Refactor

1. **Remove the entire `RenameModal` component** (~60 lines) - reuse `EditSessionModal`

2. **Remove custom context menu rendering** (~30 lines) - use Radix ContextMenu or DropdownMenu

3. **Remove manual event listeners** for click-outside and escape (~20 lines) - Radix handles this

4. **Consolidate `handleRenameSave`** - the logic is identical to `handleModalSave` in SessionListView

### Suggested Minimal Implementation

In AppSidebar.tsx - only ~30-40 new lines needed:

import { DropdownMenu, DropdownMenuContent, DropdownMenuItem, DropdownMenuTrigger } from '../ui/dropdown-menu';
import { EditSessionModal } from '../ui/EditSessionModal'; // extracted shared component

// Add state
const [renameSession, setRenameSession] = useState<Session | null>(null);

// In SessionList, wrap each session button with DropdownMenu using onContextMenu
<DropdownMenu>
  <DropdownMenuTrigger asChild onContextMenu={(e) => { e.preventDefault(); /* trigger open */ }}>
    <button>...</button>
  </DropdownMenuTrigger>
  <DropdownMenuContent>
    <DropdownMenuItem onClick={() => setRenameSession(session)}>
      <Edit2 className="w-4 h-4 mr-2" /> Rename
    </DropdownMenuItem>
  </DropdownMenuContent>
</DropdownMenu>

// Reuse the modal
<EditSessionModal
  session={renameSession}
  isOpen={!!renameSession}
  onClose={() => setRenameSession(null)}
  onSave={handleRenameSave}
/>

[zanesq]

closing in favor of #6995 for now, we may consider this later if we decide the inline option isn't working well