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/templates/ipam_allocations.html

@@ -239,8 +239,8 @@ function renderAllocations(allocations) {
                         </td>
                         <td style="padding: 0.75rem;">
                             ${a.status === 'reserved' ? 
-                                `<span class="badge badge-info" style="color: #fff;"><i class="fa-solid fa-lock"></i> ${t('reserved')}</span>` : 
-                                `<span class="badge badge-success" style="color: #fff;">✓ ${t('allocated')}</span>`}
+                                `<span class="badge badge-info"><i class="fa-solid fa-lock"></i> ${t('reserved')}</span>` : 
+                                `<span class="badge badge-success">✓ ${t('allocated')}</span>`}
                         </td>
                         <td style="padding: 0.75rem; color: var(--text-muted); font-size: 0.875rem;">${a.created_at ? new Date(a.created_at).toLocaleDateString('ru-RU') : '-'}</td>
                         <td style="padding: 0.75rem; text-align: right;">