Support generate all file changes first + unified preview then single apply/save confirmation

[id-is-null]

In multi-file tasks, Continue generates diffs file-by-file with per-file interruptions (even Accept All feels sequential as each diff pops up/highlights progressively).
Desired flow (like Cursor/Aider):

AI generates complete changes for all affected files in one pass, without interrupting per file.
Once done, show a unified preview of every diff in chat or multi-file view → user can inspect each file and check cross-file consistency/dependencies.
Finally, offer one confirmation (e.g. “Apply All & Save All” button/shortcut) to commit everything to disk — or Reject All / discard if issues found.

This “full generation → complete review → one-time apply” matches real refactoring needs far better than partial/sequential application.
Current Accept All helps but misses the “see everything first” step. Please add a config (e.g. batchGenerateFirst: true), slash command (/batch-edit), or make Edit mode support this batch-then-confirm behavior by default.
Huge UX improvement for multi-file work — thanks!