enhancement: Settings UI redesign — hierarchical layout with sub-pages

[grimmerk]

Problem

Current Settings popup changes content based on which tab is active (Projects/Sessions/Terminal). This has two issues:

1. Discoverability: first-time users may not realize Settings content changes per tab
2. Scalability: as more settings are added, the popup gets too long (already tight on Sessions tab)

Proposed design

Hierarchical layout with sub-pages:

• Settings opens a panel showing categories: General, Projects, Sessions, Terminal, Shortcuts
• Clicking a category shows its settings in a sub-page
• All settings always discoverable regardless of current tab

Notes

• Not urgent — current design works, just not ideal long-term
• Can be done incrementally (e.g., start by adding category headers that expand/collapse)

🤖 On behalf of @grimmerk — generated with Claude Code