fix(app): implement Cmd+A/Z/Shift+Z/Y editing shortcuts in framed editables

[jeonghun-jj-lee]

Summary

Extends the global editing fallback (patch #11's installGlobalClipboardFallback) to cover select-all, undo, and redo — the same class of bug where native browser actions are suppressed by the VS Code/Electron platform layer inside the sandboxed iframe.

Changes

packages/app/src/utils/global-clipboard.ts

• Added selectAll() helper: .select() for form fields, Range.selectNodeContents for contenteditables
• Extended installGlobalClipboardFallback to handle a, z, y keys in addition to v, c, x
• Cmd+A → selectAll(target)
• Cmd+Z → document.execCommand('undo')
• Cmd+Shift+Z / Cmd+Y → document.execCommand('redo')
• Relaxed the shift-key guard to allow event.shiftKey specifically for the redo chord
• Updated module header comment to reflect expanded scope

packages/app/src/utils/global-clipboard.test.ts

• 10 new tests covering all acceptance criteria (select-all in inputs/textareas/contenteditables, undo, redo, guard preservation)

Acceptance Criteria

• Cmd+A selects all text in the chat input (contenteditable)
• Cmd+C copies mouse-selected or Cmd+A-selected text to OS clipboard
• Cmd+X cuts selected text to OS clipboard
• Cmd+Z undoes the last edit in the chat input
• Cmd+Shift+Z (or Cmd+Y) redoes
• All of the above also work in other editables inside the iframe (API key fields, search inputs)
• Plain browser tab at http://127.0.0.1:{port} is unaffected (native behavior preserved)
• Existing Cmd+V paste behavior unchanged
• bun test passes, typecheck green

Closes #115