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/database.py

@@ -20,9 +20,11 @@ class Base(DeclarativeBase):
 async def init_db() -> None:
     async with engine.begin() as conn:
         await conn.run_sync(Base.metadata.create_all)
-        # Add container_mode column if not present (existing databases)
+        # Add columns introduced after initial schema (idempotent)
         with suppress(Exception):
             await conn.exec_driver_sql("ALTER TABLE nodes ADD COLUMN container_mode BOOLEAN NOT NULL DEFAULT 0")
+        with suppress(Exception):
+            await conn.exec_driver_sql("ALTER TABLE nodes ADD COLUMN custom_colors JSON")
 
 
 async def get_db():

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)

backend/app/schemas/canvas.py

@@ -21,6 +21,7 @@ class NodeSave(BaseModel):
     notes: str | None = None
     parent_id: str | None = None
     container_mode: bool = False
+    custom_colors: dict | None = None
     pos_x: float = 0
     pos_y: float = 0
 

backend/app/schemas/nodes.py

@@ -20,6 +20,7 @@ class NodeBase(BaseModel):
     pos_y: float = 0
     parent_id: str | None = None
     container_mode: bool = False
+    custom_colors: dict | None = None
 
 
 class NodeCreate(NodeBase):
@@ -41,6 +42,7 @@ class NodeUpdate(BaseModel):
     pos_x: float | None = None
     pos_y: float | None = None
     container_mode: bool | None = None
+    custom_colors: dict | None = None
 
 
 class NodeResponse(NodeBase):