feat: mention selected file lines with Cmd+I

[opsCar]

Summary

• Add Cmd+I on macOS and Ctrl+I on Windows/Linux to mention the currently selected source lines in the chat prompt.
• Reuse the existing chat-input insertion path and @mention formatting, including quoted paths with spaces and inclusive single-line or multi-line ranges.
• Map native browser selections back to logical source lines, including exact line-boundary selections and wrapped lines.
• Ignore the shortcut outside the visible Source view, inside editable controls, and for repeated keydown events.

Examples:

@components/AppShell.tsx:154
@components/FileViewer.tsx:80-120
@"project files/app.ts":3-9

Validation

• node_modules/.bin/tsc --noEmit
• npm run lint
• node --test lib/*.test.mjs components/*.test.mjs — 102 tests passed
• git diff --check origin/main...HEAD

Notes

• Preview, Diff, image, audio, and document views do not handle this shortcut because they do not expose source-line selections.
• next build was intentionally not run, following the repository's development guidance.