Add multi-plate 3MF plate selection for reprinting (Issue #93)

When reprinting a multi-plate 3MF file exported with "All sliced file",
the system now asks which plate to print and only maps filaments for
that specific plate.

Backend changes:
- Add GET /archives/{id}/plates endpoint to list available plates
  with thumbnails, print times, and per-plate filament requirements
- Add GET /archives/{id}/plate-thumbnail/{index} for plate thumbnails
- Update GET /archives/{id}/filament-requirements to accept plate_id
  query parameter for filtering filaments by plate
- Add plate_id field to ReprintRequest schema
- Update POST /archives/{id}/reprint to use plate_id from request body
  instead of auto-detecting (maintains backward compatibility)

Frontend changes:
- Add getArchivePlates API method
- Update getArchiveFilamentRequirements to accept optional plateId
- Update reprintArchive to accept plate_id in options
- Add plate selection UI to ReprintModal when multi-plate 3MF detected
- Show plate thumbnails, names, and filament counts in selection grid
- Require plate selection before printing multi-plate files
- Filter filament requirements to show only selected plate's filaments

Tests:
- Add unit tests for multi-plate slice_info.config parsing
- Add unit tests for plate detection from gcode files
- Add integration tests for new plate endpoints

backend/tests/integration/test_archives_api.py

@@ -380,3 +380,35 @@ class TestArchiveF3DEndpoints:
         assert with_f3d["f3d_path"] == "archives/test/design.f3d"
         assert without_f3d is not None
         assert without_f3d["f3d_path"] is None
+
+    # ========================================================================
+    # Multi-Plate 3MF endpoints (Issue #93)
+    # ========================================================================
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_get_archive_plates_not_found(self, async_client: AsyncClient):
+        """Verify 404 when fetching plates for non-existent archive."""
+        response = await async_client.get("/api/v1/archives/999999/plates")
+        assert response.status_code == 404
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_get_plate_thumbnail_not_found(self, async_client: AsyncClient):
+        """Verify 404 when fetching plate thumbnail for non-existent archive."""
+        response = await async_client.get("/api/v1/archives/999999/plate-thumbnail/1")
+        assert response.status_code == 404
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_filament_requirements_not_found(self, async_client: AsyncClient):
+        """Verify filament-requirements returns 404 for non-existent archive."""
+        response = await async_client.get("/api/v1/archives/999999/filament-requirements")
+        assert response.status_code == 404
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_filament_requirements_with_plate_id_not_found(self, async_client: AsyncClient):
+        """Verify filament-requirements with plate_id returns 404 for non-existent archive."""
+        response = await async_client.get("/api/v1/archives/999999/filament-requirements?plate_id=1")
+        assert response.status_code == 404