Change updated PR indicators from amber/orange to purple

[glehmann]

Yellow/amber is already used for checks-in-progress throughout the UI, making the UPDATED badge and tab status dot hard to distinguish.

Changes:

1. src/browser/components/home.tsx — UPDATED badge and Updated filter toggle: swap amber classes for purple
2. src/browser/components/app-shell.tsx — Tab status dot: swap bg-orange-500 for bg-purple-500

No data model changes, just CSS class swaps.