feat: IPAM improvements - server/node subnet binding, theme color fixes

- IPAM link-allocations: filter networks by proxmox_server_id and proxmox_node
  to prevent cross-server/cross-node IP assignment
- IPAM link-allocations: add manual server+network selection modal before linking
- IPAM add-network forms: add Proxmox server and node selectors with dynamic
  cluster-info loading (ipam_dashboard.html, ipam_networks.html)
- models.py / schemas.py: add proxmox_node field to IPAMNetwork
- migrations: migration #16 adds proxmox_node column to ipam_networks table
- Theme fixes: replace all hardcoded rgba/color:#fff/color:white with CSS vars
  across base.html, virtual_machines.html, instance_detail.html,
  os_templates.html, proxmox_vms.html, ipam_network_detail.html,
  ipam_allocations.html
- i18n: add proxmox_node, node selector, link-allocations modal keys (ru/en)
- Version bump: 1.0.4 -> 1.0.5

backend/app/schemas.py

@@ -359,6 +359,7 @@ class IPAMNetworkBase(BaseModel):
     dns_secondary: Optional[str] = Field(None, description="Secondary DNS server")
     dns_domain: Optional[str] = Field(None, max_length=255, description="DNS domain")
     proxmox_server_id: Optional[int] = Field(None, description="Associated Proxmox server ID")
+    proxmox_node: Optional[str] = Field(None, max_length=100, description="Proxmox node name (e.g., pve1)")
     proxmox_bridge: Optional[str] = Field(None, max_length=20, description="Proxmox bridge (e.g., vmbr0)")
     is_active: bool = Field(default=True, description="Is network active")
 
@@ -400,6 +401,7 @@ class IPAMNetworkUpdate(BaseModel):
     dns_secondary: Optional[str] = None
     dns_domain: Optional[str] = Field(None, max_length=255)
     proxmox_server_id: Optional[int] = None
+    proxmox_node: Optional[str] = Field(None, max_length=100)
     proxmox_bridge: Optional[str] = Field(None, max_length=20)
     is_active: Optional[bool] = None