Add Find in Files (Ctrl+Shift+F)

[sbaker]

Summary

Add workspace-wide text search (Find in Files) similar to VS Code's Ctrl+Shift+F functionality.

Current behavior

• Ctrl+F searches within the current file only
• No way to search across all files in the workspace

Proposed behavior

• Ctrl+Shift+F opens a search panel in the sidebar
• Searches all files in the open workspace
• Results grouped by file with line previews
• Click result to open file at that line

References

• Current find: Monaco editor built-in (Ctrl+F)
• Hotkeys: frontend/src/modules/types/hotkeys.ts