Add JSON import feature with four-phase validation

[rburman-tech]

Closes the round-trip gap opened by #268 (export now includes routes, layers, factions).

What this adds

• Import button in the sector sidebar (owners only, not shared views)
• "Download for Import" button in the import modal that exports raw JSON (unlocalized keys, required for reimport)
• Four-phase validator that runs before any Firestore write:
  • Phase 1: top-level structure
  • Phase 2: per-entity schema and attribute enum values
  • Phase 3: ID-reference graph (parent chains, orphan detection, cycle detection, faction asset locations)
  • Phase 4: coordinate bounds and hex collision detection
• Scrollable error list in the UI so users see all problems at once
• 40 unit tests covering every phase and failure mode

Design notes

The validator is a pure function (no Firestore reads) so it can be tested exhaustively and fails cleanly before touching any data. Raw JSON format is required for import because translateEntities localizes attribute values — reimporting a translated export would corrupt the app.