Import: replace 'replace existing' toggle with 4 reconciliation modes

[hefler]

Summary

• Replaces the single "replace existing variables" boolean toggle on the JSON import view with four explicit modes (ImportMode enum):
  • Merge — create missing collections/modes/variables, update matches by name. Nothing is ever deleted.
  • Update only — touch only what already exists in both the file and the document; nothing created, nothing deleted.
  • Merge and delete anything not in the file (Sync) — merge first, then delete anything anywhere in the document that isn't in the file, including whole collections the file never mentions. Matches are updated in place (not deleted+recreated), so their component bindings survive.
  • Clean import — today's existing behaviour: wipe every local collection, then import fresh. Every variable is recreated, so every existing component binding breaks, even where the recreated variable matches exactly.
• Confirmation dialog now only gates Sync and Clean (the two modes that can delete something); its copy is tailored per mode.
• ImportSummary gained variablesDeleted / modesDeleted / collectionsDeleted, surfaced in the import summary panel.

Test plan

• npm run tsc
• npm run build
• Manual check in Figma: run each of the 4 modes against a document with existing variables/components bound to them, confirm bindings survive for Merge/Update only/Sync-matches and break only where expected