Fix select chevrons and migrate dropdowns to AppSelect

[Modsofthenation]

Summary

Dropdown chevrons were hugging the right edge of inputs. This PR:

1. Introduces themed AppSelect (react-select) used across the UI — searchable menus, padded indicator, PRM colors
2. Keeps a native <select> CSS fallback with appearance: none and a custom chevron inset via padding-right: 2.5rem / background-position: right 0.85rem

Test plan

• pnpm --filter @prm/ui typecheck
• Home → Cycle select: chevron has breathing room; menu opens with themed options
• Spot-check Settings / Chat / Cycles / Roles selects
• Light + dark themes both look correct

  

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