Add device_type field, mobile-friendly improvements, and device filtering

- Device model: add device_type column (String 50, nullable)
- Device add/edit: type select with presets + custom entry
- Device detail: show type in info card; new Edit Device form
- Device list: Type column + client-side filter bar (type + text search)
- Mobile: card-style responsive tables on dashboard and device list,
  form-grid-2col collapse, larger tap targets, stacked form-actions,
  column picker viewport fix, filter bar full-width controls
- Assign page: larger radio touch targets (min-height 44px)
- 3 new acceptance tests for device_type (45 total)

models.py

@@ -10,6 +10,7 @@ class Device(Base):
     id = Column(Integer, primary_key=True, autoincrement=True)
     name = Column(String(100), nullable=False, unique=True)
     battery_slots = Column(Integer, nullable=False, default=1)
+    device_type   = Column(String(50),  nullable=True)
     notes = Column(Text, nullable=True)
 
     batteries = relationship("Battery", back_populates="device")