feat: add file creation from FileTree context menu

[rjroy]

Summary

• Add "Create File" option to directory context menu in FileTree
• Opens dialog for entering file name (alphanumeric with - and _ only)
• Creates empty markdown file with .md extension added automatically
• Follows same pattern as existing "Add Directory" feature

Closes #332

Test plan

• Right-click a directory in Recall tab, select "Create File"
• Enter a valid file name (e.g., my-new-note)
• Verify empty .md file is created in the selected directory
• Verify directory listing refreshes to show new file
• Test validation rejects names with spaces or special characters
• Test duplicate file name shows error

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 76.57143% with 41 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
backend/src/handlers/browser-handlers.ts	0.00%	26 Missing ⚠️
frontend/src/components/BrowseMode.tsx	46.66%	8 Missing ⚠️
frontend/src/components/FileTree.tsx	86.79%	7 Missing ⚠️

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