add confirmation dialogs for discard all variants

[rafavalls]

Summary

• Adds a confirmation AlertDialog before removing all section variants, preventing accidental destructive actions
• Adds a "Discard all page variants" button with its own confirmation dialog to the page-level variant list header
• Implements handleRemoveAllPageVariants which unwraps the multivariate page structure back to the default (always) variant and auto-saves

Test plan

• Open a page with section-level variants, click the trash icon — confirm dialog appears before removal
• Open a page with page-level variants, click the trash icon in the page variants header — confirm dialog appears, accepting reverts to single-version page and auto-saves
• Cancel in either dialog leaves variants untouched

🤖 Generated with Claude Code

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Summary by cubic

Add confirmation dialogs before discarding section or page variants to prevent accidental deletions. Adds a “Discard all page variants” action that unwraps to the default variant and auto-saves with a preview refresh.

• New Features

  • Confirmation dialog when removing all section variants.
  • “Discard all page variants” button with confirmation in the page variants list header.
  • handleRemoveAllPageVariants unwraps to the default variant (prefers “always” rule, falls back to first), clears selection, auto-saves, and refreshes preview.

• Refactors

  • Reformatted panel/header markup after adding a fragment wrapper (no behavior change).

^{Written for commit 537d664. Summary will update on new commits.}