Replace emoji indicators with custom tinted menubar icons

[kbsooo]

Summary

• Replace generic 🟢🟡🔴 emoji status indicators with custom PNG icons that are tinted green/yellow/red based on usage level
• Two icon styles available: Spark (Claude logo) and Clawd (pixel art character), selectable via a segmented picker in the popover footer
• Icons are Base64-embedded in source code for build-system independence (works with both Xcode and SPM)

Changes

• New: ClaudeUsage/StatusIcon.swift — icon style enum, Base64 image data, tinting utilities (.sourceAtop for silhouette, .color blend for multi-color)
• Modified: ClaudeUsageApp.swift — updateStatusItem() uses button.image + button.title instead of emoji text
• Modified: UsageManager.swift — added statusUtilization computed property
• Modified: UsageView.swift — added Spark/Clawd icon picker in popover footer
• New: icon.png, clawd.png — source icon assets

Screenshots

Spark (green)	Clawd (green)
Uses Claude spark logo silhouette	Uses Clawd pixel art character

Test plan

• Build and run — verify colored icon + percentage appears in menubar
• Toggle between Spark and Clawd in popover footer — icon updates immediately
• Verify all three color states: green (<70%), yellow (70-89%), red (≥90%)
• Verify loading (⏳) and error (❌) states display correctly
• Test in both light and dark menubar appearance

🤖 Generated with Claude Code