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/migrations/migrations.py

@@ -1035,6 +1035,20 @@ def migrate_backup_jobs(conn):
     logger.info("✓ backup_jobs table created")
 
 
+# ==================== Migration 16: IPAM Network Node ====================
+
+def migrate_ipam_network_node(conn):
+    """Add proxmox_node column to ipam_networks table"""
+    if not table_exists(conn, 'ipam_networks'):
+        logger.info("Table ipam_networks does not exist, skipping")
+        return
+
+    if add_column_if_not_exists(conn, 'ipam_networks', 'proxmox_node', 'VARCHAR(100)'):
+        logger.info("✓ Added proxmox_node column to ipam_networks")
+    else:
+        logger.info("✓ ipam_networks.proxmox_node already exists")
+
+
 def run_all_migrations(engine, db_session=None):
     """
     Run all migrations in order.
@@ -1169,6 +1183,14 @@ def run_all_migrations(engine, db_session=None):
                 logger.warning(f"Backup jobs migration: {e}")
                 conn.rollback()
 
+            # Migration 16: IPAM Network Node
+            try:
+                migrate_ipam_network_node(conn)
+                conn.commit()
+            except Exception as e:
+                logger.warning(f"IPAM network node migration: {e}")
+                conn.rollback()
+
         logger.info("=" * 50)
         logger.info("✓ All migrations completed successfully")
         logger.info("=" * 50)