feat: complete node grouping with visual indicators and clustering

[lowhung]

Summary

Complete the node grouping feature with visual clustering and a group legend.

Changes

Visual Indicators

• Color palette: Added 8 distinct colors for groups (steel blue, light green, pink, peach, plum, powder blue, yellow, lavender)
• Group rings: Nodes in a group display a colored ring around them
• Group legend: Right panel shows active groups with their colors and node counts

Layout Clustering

• Group attraction force: Nodes in the same group attract each other slightly in force-directed layout
• Groups that are collapsed don't apply attraction forces

Group Management

• Groups use palette colors automatically when created
• Groups can be collapsed/expanded via the group panel
• Collapsed groups show "(collapsed)" in the legend

Files Changed

• src/ui/types.rs: Added GROUP_COLORS palette and get_group_color() function
• src/ui/drawing.rs: Added group ring rendering around nodes
• src/ui/layout.rs: Added group attraction force to force-directed layout
• src/ui/panels.rs: Added draw_group_legend() function, updated group creation to use palette
• src/ui/app.rs: Pass groups to layout and DrawContext, show group legend

Testing

• All 90 existing tests pass
• Layout tests updated to pass groups parameter

Closes #21