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():