feat: add Docker node type, group rect border style, and categorized node type selector

- Fix DockerNode default icon: use Anchor instead of Package
- Add border_style option to Group Rectangle (solid/dashed/dotted/double/none)
- Group node type selector by category (Hardware, Virtualization, IoT, Generic)
- Add tests for border style feature

frontend/src/utils/themes.ts

@@ -56,6 +56,7 @@ export const THEMES: Record<ThemeId, ThemePreset> = {
         printer:  { border: '#8b949e', icon: '#8b949e' },
         computer: { border: '#a855f7', icon: '#a855f7' },
         cpl:      { border: '#e3b341', icon: '#e3b341' },
+        docker:   { border: '#2496ED', icon: '#2496ED' },
         generic:  { border: '#8b949e', icon: '#8b949e' },
         groupRect:{ border: '#00d4ff', icon: '#00d4ff' },
       },
@@ -109,6 +110,7 @@ export const THEMES: Record<ThemeId, ThemePreset> = {
         printer:  { border: '#94a3b8', icon: '#94a3b8' },
         computer: { border: '#c084fc', icon: '#c084fc' },
         cpl:      { border: '#fbbf24', icon: '#fbbf24' },
+        docker:   { border: '#2496ED', icon: '#2496ED' },
         generic:  { border: '#94a3b8', icon: '#94a3b8' },
         groupRect:{ border: '#22d3ee', icon: '#22d3ee' },
       },
@@ -162,6 +164,7 @@ export const THEMES: Record<ThemeId, ThemePreset> = {
         printer:  { border: '#6b7280', icon: '#6b7280' },
         computer: { border: '#7c3aed', icon: '#7c3aed' },
         cpl:      { border: '#b45309', icon: '#b45309' },
+        docker:   { border: '#2496ED', icon: '#2496ED' },
         generic:  { border: '#6b7280', icon: '#6b7280' },
         groupRect:{ border: '#0284c7', icon: '#0284c7' },
       },
@@ -215,6 +218,7 @@ export const THEMES: Record<ThemeId, ThemePreset> = {
         printer:  { border: '#8888ff', icon: '#8888ff' },
         computer: { border: '#ff00ff', icon: '#ff00ff' },
         cpl:      { border: '#ffff00', icon: '#ffff00' },
+        docker:   { border: '#00aaff', icon: '#00aaff' },
         generic:  { border: '#8888ff', icon: '#8888ff' },
         groupRect:{ border: '#00ffff', icon: '#00ffff' },
       },
@@ -268,6 +272,7 @@ export const THEMES: Record<ThemeId, ThemePreset> = {
         printer:  { border: '#005500', icon: '#005500' },
         computer: { border: '#008822', icon: '#008822' },
         cpl:      { border: '#66ff33', icon: '#66ff33' },
+        docker:   { border: '#00cc88', icon: '#00cc88' },
         generic:  { border: '#006600', icon: '#006600' },
         groupRect:{ border: '#00ff41', icon: '#00ff41' },
       },