fix(chat): support Home and End in the composer

[Zhangs-11]

Summary

• implement consistent Home and End behavior in the multiline chat composer
• use the current logical line for unmodified shortcuts and the whole input for Ctrl/Cmd shortcuts
• preserve native Shift selection direction and leave IME composition and Alt+Home/Alt+End untouched

Behavior

Shortcut	Result
Home / End	Move to the start / end of the current logical line
Shift+Home / Shift+End	Extend the selection to the current line boundary
Ctrl/Cmd+Home / Ctrl/Cmd+End	Move to the start / end of the full message
Ctrl/Cmd+Shift+Home / Ctrl/Cmd+Shift+End	Extend the selection to the full-message boundary

Testing

• npm test — 883 tests passed
• node_modules/.bin/tsc --noEmit
• npm run lint
• Chromium browser verification covering multiline caret movement and Shift selection on the rebased commit

Demo

Recorded from Zhangs-11/pi-web@60d3908fa72e1c4d2bc1e7ba8f324319f9f82c6c on the clean rebased branch, served at http://127.0.0.1:30154 with next dev because repository instructions prohibit next build during development. The run used a fresh isolated Pi agent directory, workspace, and Chrome profile with the normal application transport. A real GPT-6-Astra round created the displayed session; the demonstrated key presses were normal browser keyboard input with no fixtures, mock transport, DOM-event injection, or test-only hooks.

[agegr]

Thanks for the contribution. These shortcuts already work through the native textarea on main, so we will keep the native behavior and close this PR. A specific OS/browser reproduction of a failure on main would help justify a targeted fix.