feat: per-node custom color styling (border, background, icon)

- Add resolveNodeColors() utility merging type defaults with per-node overrides
- Default colors per node type (cyan=isp/router/lxc/ap, green=switch/nas,
  purple=server/vm, orange=proxmox, amber=iot, gray=generic)
- Remove glowColor prop from BaseNode — colors now come from node data
- ProxmoxGroupNode uses resolveNodeColors for group border/header/icon
- NodeModal: Appearance section with 3 color swatches (border, background, icon)
  — click swatch to open native color picker; Reset to defaults button
- custom_colors persisted as JSON in DB (backend model + schemas + migration)
- 7 new unit tests for resolveNodeColors covering all node types + partial overrides

backend/app/db/models.py

@@ -34,6 +34,7 @@ class Node(Base):
     pos_y: Mapped[float] = mapped_column(Float, default=0)
     parent_id: Mapped[str | None] = mapped_column(String, ForeignKey("nodes.id"))
     container_mode: Mapped[bool] = mapped_column(Boolean, default=False)
+    custom_colors: Mapped[dict | None] = mapped_column(JSON, nullable=True)
     last_seen: Mapped[datetime | None] = mapped_column(DateTime(timezone=True))
     response_time_ms: Mapped[int | None] = mapped_column(Integer)
     created_at: Mapped[datetime] = mapped_column(DateTime(timezone=True), default=_now)