Add click-to-deselect conversation functionality

[mmenanno]

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• I have read the README and Contributor Guidelines
• I have signed the Contributor Licence Agreement

Contributor checklist:

• My contribution is not related to translations.
• My commits are in nice logical chunks with good commit messages
• My changes are rebased on the latest main branch
• A pnpm run ready run passes successfully (more about tests here)
• My changes are ready to be shipped to users

Description

This PR adds click-to-deselect conversation functionality to the Signal Desktop LeftPane, providing users with an intuitive way to clear conversation selections by clicking empty areas. Semi-relevant issue here #1784

User Value

• Improved UX: Users can now click empty areas in the conversation list to deselect conversations
• Intuitive behavior: Matches user expectations from other desktop applications
• Mouse alternative: Provides click-based alternative to the Cmd+Shift+C keyboard shortcut
• Non-disruptive: Preserves all existing functionality for interactive elements

What Changed

• Added handleLeftPaneClick event handler to LeftPane.tsx
• Smart detection of empty areas vs interactive elements using event.target.closest()
• Calls showConversation({ conversationId: undefined, messageId: undefined }) to deselect
• Added comprehensive unit tests in LeftPane_test.ts

Technical Implementation

• Uses event.target.closest() to detect clicks on buttons, inputs, conversation items, etc.
• Only deselects when clicking truly empty areas (not on interactive elements)
• Includes preventDefault() and stopPropagation() for clean event handling
• Follows existing Signal Desktop patterns for event handling and component structure
• Minimal code footprint (~20 lines) with no performance impact

Test Approach

Manual Testing:

• ✅ Verified click-to-deselect works in empty areas of conversation list
• ✅ Verified clicking buttons/conversation items preserves normal behavior
• ✅ Verified Cmd+Shift+C keyboard shortcut still works as expected
• ✅ Tested edge cases (no conversation selected, various click targets)
• ✅ Tested in development environment following CONTRIBUTING.md setup instructions

Automated Testing:

• ✅ Added comprehensive unit tests covering:
  • DOM element detection (empty vs interactive areas)
  • Deselection condition logic
  • API call format verification
  • Edge case coverage (no selection, interactive elements)
• ✅ All existing tests continue to pass (1,236 tests total)

Operating Systems:

• ✅ macOS 14.6.0 (darwin 24.6.0) - Primary testing platform

Additional Testing:

• ✅ Code formatted with Prettier
• ✅ TypeScript compilation successful
• ✅ ESLint accessibility rules properly handled with appropriate comments