Feature: Add Gemini 3 Support and Comprehensive Lint Cleanup

[Dhineshkumaran]

Problem Description

The application currently lacks explicit context window limits for the newer Gemini models (gemini-3-flash-preview, gemini-2.5-pro, etc.), causing them to fall back to default, inaccurate token limits.

Additionally, the codebase has accumulated several linting warnings and errors that need to be addressed to maintain code quality, specifically:

• Missing explicit return types in many React components and functions.
• A "React Fast Refresh" warning caused by exporting helper functions alongside components (WorkspacePicker).
• Unused variables in some backend handlers.

Proposed Solution

1. Update Context Awareness: Add explicit configuration for Gemini 3 and 2.5 models in ContextUsageIndicator.
2. Refactor Components: Move the selectWorkspaceFolder helper out of WorkspacePicker.tsx to a new utility file to resolve Fast Refresh warnings.
3. Linting Pass: Perform a codebase-wide cleanup to add missing return types (: void, : React.JSX.Element, etc.) and fix unused variable warnings.

Task List

• Add gemini-3-flash-preview and gemini-2.5-* limits to ContextUsageIndicator.tsx.
• Refactor WorkspacePicker.tsx to move logic to lib/workspace-utils.ts.
• Add explicit return types to:
  • ChatContainer, ModelSwitcher, ToolCallRenderer.
  • ImageViewer, PDFViewer, TabBar.
  • src/main/ipc/threads.ts.
• Fix unused variables in src/main/storage.ts and thread-context.tsx.