feat(diagnostic, archives): install step 4 — proactive check + reactive banner

Two complementary surfaces for the most-missed install step ("Store sent
  files on external storage"):

  1. Connection diagnostic check (printer-side variant)
     - Reads state.store_to_sdcard, parsed from MQTT home_flag bit 11.
     - Pass / fail / skip; instant, no I/O.
     - Catches the newer-firmware variant where the toggle moved onto the
       printer itself (P2S 01.02 / Studio 2.6+).

     An FTP upload-and-verify probe was tried first and rejected. /cache
     is always writable from Bambuddy regardless of the slicer setting;
     only BambuStudio's own behaviour changes when the toggle flips.
     Empirically confirmed against X1C + H2D with the slicer option
     toggled off: probe still succeeded, home_flag bit 11 stayed True.

  2. Archives-page banner (slicer-side variant)
     - The slicer-side toggle is invisible to the printer — older
       BambuStudio doesn't push the change to the printer. The diagnostic
       can't see it.
     - Symptom is deterministic: archiver creates rows with
       extra_data.no_3mf_available=True (main.py:2770) when it can't pull
       the 3MF from /cache after a slicer-initiated print.
     - New endpoint GET /archives/no-3mf-warning returns whether any
       archive in the last 30 days has the flag (excluding soft-deleted).
     - Amber dismissible banner at the top of /archives; one-shot
       localStorage dismissal (matches Layout.tsx update-banner pattern,
       but persistent across sessions).
     - React-Query disabled after dismissal so the endpoint isn't polled
       once the user has been told.

backend/tests/integration/test_archives_api.py

@@ -534,6 +534,96 @@ class TestArchivesAPI:
         assert "successful_prints" in result
 
 
+class TestNo3MFWarning:
+    """`GET /archives/no-3mf-warning` — install step 4 reactive nudge.
+
+    The connection diagnostic's external_storage check only catches the
+    printer-side variant of the setting (newer firmware). For older slicers
+    where the toggle lives only in BambuStudio, the printer never reports
+    it. The fallback path in main.py creates the archive with
+    extra_data.no_3mf_available=True; this endpoint exposes that as a
+    boolean so the frontend can surface a one-time banner.
+    """
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_returns_true_when_recent_fallback_exists(
+        self, async_client: AsyncClient, archive_factory, printer_factory, db_session
+    ):
+        printer = await printer_factory()
+        await archive_factory(printer.id, extra_data={"no_3mf_available": True})
+
+        response = await async_client.get("/api/v1/archives/no-3mf-warning")
+
+        assert response.status_code == 200
+        assert response.json() == {"has_fallback": True}
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_returns_false_when_no_archives(self, async_client: AsyncClient):
+        response = await async_client.get("/api/v1/archives/no-3mf-warning")
+
+        assert response.status_code == 200
+        assert response.json() == {"has_fallback": False}
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_returns_false_when_only_normal_archives(
+        self, async_client: AsyncClient, archive_factory, printer_factory, db_session
+    ):
+        printer = await printer_factory()
+        # extra_data has other keys but no_3mf_available is absent — normal
+        # archives must not trigger the nudge.
+        await archive_factory(printer.id, extra_data={"makerworld_url": "https://example"})
+        await archive_factory(printer.id, extra_data=None)
+
+        response = await async_client.get("/api/v1/archives/no-3mf-warning")
+
+        assert response.status_code == 200
+        assert response.json() == {"has_fallback": False}
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_ignores_archives_older_than_30_days(
+        self, async_client: AsyncClient, archive_factory, printer_factory, db_session
+    ):
+        from datetime import datetime, timedelta, timezone
+
+        from backend.app.models.archive import PrintArchive
+
+        printer = await printer_factory()
+        archive = await archive_factory(printer.id, extra_data={"no_3mf_available": True})
+        # Backdate past the 30-day window — old fallbacks are forgiven.
+        archive.created_at = datetime.now(timezone.utc) - timedelta(days=45)
+        await db_session.commit()
+
+        response = await async_client.get("/api/v1/archives/no-3mf-warning")
+
+        assert response.status_code == 200
+        assert response.json() == {"has_fallback": False}
+        # Sanity: row really is in the DB, we just don't surface it.
+        assert (await db_session.get(PrintArchive, archive.id)) is not None
+
+    @pytest.mark.asyncio
+    @pytest.mark.integration
+    async def test_ignores_soft_deleted_fallbacks(
+        self, async_client: AsyncClient, archive_factory, printer_factory, db_session
+    ):
+        from datetime import datetime, timezone
+
+        printer = await printer_factory()
+        archive = await archive_factory(printer.id, extra_data={"no_3mf_available": True})
+        archive.deleted_at = datetime.now(timezone.utc)
+        await db_session.commit()
+
+        response = await async_client.get("/api/v1/archives/no-3mf-warning")
+
+        assert response.status_code == 200
+        # Soft-deleted fallbacks have been actioned (user clearing the
+        # evidence). Stop nudging.
+        assert response.json() == {"has_fallback": False}
+
+
 class TestPrintLogEntryDelete:
     """#1687: per-row delete on the Print Log page.