Add inline assign from dashboard, specific battery picker on device, dynamic install rows

- Dashboard: replace Assign link with device dropdown + arrow button for
  quick inline assignment without leaving the page
- Device detail: replace hardcoded 4-row install form with 1 row + '+ Add
  brand' button that clones rows dynamically
- Device detail: add 'Install Specific Battery' card with dropdown of all
  available batteries (label, brand, size, notes) via new /device/<id>/install-one route
- Tests: 4 new acceptance tests covering dashboard quick-assign and
  install-one, including capacity enforcement on both paths (39 total)

app.py

@@ -33,8 +33,9 @@ def create_app(config_object="config"):
             .filter(Battery.storage_location.isnot(None))
             .distinct().order_by(Battery.storage_location).all()
         ]
+        devices = db.query(Device).order_by(Device.name).all()
         return render_template("dashboard.html", batteries=batteries,
-                               storage_locations=storage_locations)
+                               storage_locations=storage_locations, devices=devices)
 
     # ------------------------------------------------------------------ #
     # Battery — add
@@ -361,7 +362,11 @@ def create_app(config_object="config"):
         if device is None:
             abort(404)
         brands = [r[0] for r in db.query(Battery.brand).distinct().order_by(Battery.brand).all()]
-        return render_template("device_detail.html", device=device, brands=brands)
+        available_batteries = (db.query(Battery)
+            .filter_by(status="available")
+            .order_by(Battery.label).all())
+        return render_template("device_detail.html", device=device, brands=brands,
+                               available_batteries=available_batteries)
 
     # ------------------------------------------------------------------ #
     # Devices — install batteries
@@ -436,6 +441,40 @@ def create_app(config_object="config"):
         )
         return redirect(url_for("device_detail", device_id=device_id))
 
+    # ------------------------------------------------------------------ #
+    # Devices — install one specific battery
+    # ------------------------------------------------------------------ #
+
+    @app.route("/device/<int:device_id>/install-one", methods=["POST"])
+    def device_install_one(device_id):
+        device = db.get(Device, device_id)
+        if device is None:
+            abort(404)
+        battery_id = request.form.get("battery_id", type=int)
+        battery = db.get(Battery, battery_id)
+        if battery is None or not battery.is_available():
+            flash("Battery not found or not available.", "error")
+            return redirect(url_for("device_detail", device_id=device_id))
+        if device.installed_count() >= device.battery_slots:
+            flash(
+                f"{device.name} is full "
+                f"({device.battery_slots}/{device.battery_slots} slots used).",
+                "error",
+            )
+            return redirect(url_for("device_detail", device_id=device_id))
+        existing_brands = device.installed_brands()
+        if existing_brands and battery.brand not in existing_brands:
+            flash(
+                f"Warning: {device.name} already has batteries from "
+                f"{', '.join(existing_brands)}. Mixing brands is not recommended.",
+                "warning",
+            )
+        battery.status = "installed"
+        battery.device_id = device.id
+        db.commit()
+        flash(f"{battery.label} installed in {device.name}.", "success")
+        return redirect(url_for("device_detail", device_id=device_id))
+
     # ------------------------------------------------------------------ #
     # Devices — delete
     # ------------------------------------------------------------------ #