Add bulk install-in-device from dashboard and unretire action

- Dashboard bulk toolbar: select batteries, pick a device, click 'Install
  in device'; confirms before moving already-installed batteries; enforces
  slot capacity and warns on brand mix
- Battery detail: 'Unretire Battery' button replaces 'Retire Battery' when
  battery is retired, restoring it to available status
- Tests: 3 new bulk-install-device tests (capacity block, move, success);
  42 total passing

tests/test_acceptance.py

@@ -294,6 +294,45 @@ def test_device_install_over_capacity(client):
     assert b"slot" in resp.data.lower()
 
 
+def test_bulk_install_device(client):
+    """Select multiple available batteries and install them into a device."""
+    client.post("/device/add", data={"name": "Box", "battery_slots": "3"})
+    client.post("/battery/add", data={"brand": "Eneloop", "count": "2"})
+    resp = client.post("/battery/bulk-action",
+                       data={"battery_ids": ["1", "2"], "action": "install_device",
+                             "device_id": "1"},
+                       follow_redirects=True)
+    assert resp.status_code == 200
+    assert b"Installed 2" in resp.data
+
+
+def test_bulk_install_device_over_capacity(client):
+    """Bulk install is blocked when device lacks free slots."""
+    client.post("/device/add", data={"name": "Box", "battery_slots": "1"})
+    client.post("/battery/add", data={"brand": "Eneloop", "count": "2"})
+    resp = client.post("/battery/bulk-action",
+                       data={"battery_ids": ["1", "2"], "action": "install_device",
+                             "device_id": "1"},
+                       follow_redirects=True)
+    assert resp.status_code == 200
+    assert b"slot" in resp.data.lower()
+
+
+def test_bulk_install_device_moves_installed_battery(client):
+    """Bulk install moves a battery already installed in another device."""
+    client.post("/device/add", data={"name": "Box A", "battery_slots": "2"})
+    client.post("/device/add", data={"name": "Box B", "battery_slots": "2"})
+    client.post("/battery/add", data={"brand": "Eneloop", "count": "1"})
+    client.post("/battery/1/assign", data={"device_id": "1"})
+    resp = client.post("/battery/bulk-action",
+                       data={"battery_ids": ["1"], "action": "install_device",
+                             "device_id": "2"},
+                       follow_redirects=True)
+    assert resp.status_code == 200
+    assert b"Installed 1" in resp.data
+    assert b"Box B" in client.get("/battery/1").data
+
+
 # ------------------------------------------------------------------ #
 # Dashboard — quick-assign
 # ------------------------------------------------------------------ #