Add protection/warning for personal corpus deletion

[JSv4]

Context

From PR #834 code review - users can delete their personal "My Documents" corpus without any special warning or protection.

Current Behavior

• User deletes personal corpus → documents in it become inaccessible
• Next upload → new personal corpus is auto-created (works correctly)
• No warning that this is a special corpus

Potential Improvements

Option A: Prevent deletion entirely

Add check in delete mutation/method:

if corpus.is_personal:
    raise ValidationError("Cannot delete personal corpus")

Option B: Soft warning in UI

Frontend displays warning when attempting to delete a corpus where is_personal=True.

Option C: Document current behavior

Current behavior (auto-recreate on next upload) is reasonable - just needs documentation so users understand.

Recommendation

Option B (warning) provides the best UX without being overly restrictive. Users should be able to delete if they really want to, but should understand the implications.

Priority

Low - edge case, current behavior is safe (just potentially confusing).