feat(sidebar): scan a single folder from the folder context menu (#1108)

The Refresh menu can scope a scan to one model root, but the folder a user is
looking at lives in the sidebar's unified tree, which merges every root into one
relative-path namespace. "Scan this folder" therefore addresses the folder, not a
root: the backend walks that relative path under every root that holds it, which
is also what makes the action safe while another drive is switched off.

Backend:
* GET /scan accepts `folder=<rel>` (alone or with `roots=`), rejected together
  with full_rebuild=true like the roots parameter. Validation reuses
  normalize_relative_folder(), extracted to module scope from ModelMoveService so
  the folder operations and the scan endpoint reject the same input (absolute
  paths, drive letters, `..` climbing) instead of each carrying its own copy.
* The reconcile summary carries `scope_label` (the folder) for a folder scope, so
  the result toast names the folder the user clicked instead of the roots it
  happens to live under; the completed WS payload carries it too.
* `folder` is a scope prefix exactly like a root: only that subtree is re-read or
  pruned, and an unreachable root keeps the entries that fall inside it.

Frontend:
* The sidebar folder context menu gains "Scan this folder" above "Check for
  updates in this folder" (they share the refresh divider); the entry is gated by
  the same supportsFolderManagement flag as the other folder operations.
* SidebarManager.scanFolder() resolves the node through the existing
  _resolveFolderCandidates() before doing anything: a folder no root holds any
  more explains itself ("no longer exists on disk") instead of scanning nothing,
  and an unresolvable multi-root node is refused rather than guessed.
* PageControls.refreshModels() and BaseModelApi.refreshModels() forward the folder
  scope, and _showRefreshSummary() prefers scope_label over the walked roots.

Verified live on the three-root sandbox with one drive switched off:
GET /scan?folder=pack000 walks drive-G and drive-Y, reports scope_label=pack000,
keeps drive-Z's 6 entries under that folder (kept_unreachable=6) and leaves all
420 models cached. 3704 passed, 7 skipped; frontend 1495 passed (150 files); vue
widgets 96 passed. The 2 new sidebar keys are [TODO: Translate] placeholders
pending the feature owner's go-ahead.

tests/routes/test_model_query_handler.py

@@ -307,6 +307,65 @@ async def test_scan_models_rejects_roots_with_full_rebuild():
     assert service.received_scope == "not-called"
 
 
+@pytest.mark.asyncio
+async def test_scan_models_accepts_folder_param():
+    service = ScopedScanService(roots=["/mnt/a", "/mnt/b"], summary=SUMMARY)
+    handler = ModelQueryHandler(service=service, logger=logging.getLogger(__name__))
+
+    response = await handler.scan_models(
+        SimpleNamespace(query=QueryParams({"folder": "pack\\sub"}))
+    )
+
+    assert response.status == 200
+    assert service.received_scope is not None
+    # Backslashes are normalized and no root is pinned: the backend walks this
+    # relative folder under every root that holds it.
+    assert service.received_scope.folder == "pack/sub"
+    assert service.received_scope.roots is None
+
+
+@pytest.mark.asyncio
+async def test_scan_models_rejects_absolute_folder():
+    service = ScopedScanService(roots=["/mnt/a"], summary=SUMMARY)
+    handler = ModelQueryHandler(service=service, logger=logging.getLogger(__name__))
+
+    for folder in ("/mnt/a/pack", "../pack", "C:/models/pack"):
+        response = await handler.scan_models(
+            SimpleNamespace(query=QueryParams({"folder": folder}))
+        )
+        assert response.status == 400, folder
+        assert "error" in json.loads(response.text)
+
+    assert service.received_scope == "not-called"
+
+
+@pytest.mark.asyncio
+async def test_scan_models_rejects_folder_with_full_rebuild():
+    service = ScopedScanService(roots=["/mnt/a"], summary=SUMMARY)
+    handler = ModelQueryHandler(service=service, logger=logging.getLogger(__name__))
+
+    response = await handler.scan_models(
+        SimpleNamespace(query=QueryParams({"folder": "pack", "full_rebuild": "true"}))
+    )
+
+    assert response.status == 400
+    assert service.received_scope == "not-called"
+
+
+@pytest.mark.asyncio
+async def test_scan_models_combines_folder_and_roots():
+    service = ScopedScanService(roots=["/mnt/a", "/mnt/b"], summary=SUMMARY)
+    handler = ModelQueryHandler(service=service, logger=logging.getLogger(__name__))
+
+    response = await handler.scan_models(
+        SimpleNamespace(query=QueryParams({"folder": "pack", "roots": ["/mnt/b"]}))
+    )
+
+    assert response.status == 200
+    assert service.received_scope.folder == "pack"
+    assert service.received_scope.roots == ("/mnt/b",)
+
+
 @pytest.mark.asyncio
 async def test_get_model_roots_reports_details():
     service = ScopedScanService(roots=["/mnt/a", "/mnt/b"])