Add dark mode support with Settings appearance picker

[anamhira47]

Summary

Adds full dark mode support to SweepAlert iOS by removing the hard-coded light appearance lock, introducing adaptive brand colors, and adding an in-app Appearance control on Settings (visible to signed-out users).

Changes

• Removed .preferredColorScheme(.light) so the app can follow system or user preference
• Added SweepColors theme tokens with asset-catalog adaptive colors:
  • SweepInk — primary text
  • SweepMuted — secondary text
  • SweepCardStroke — card borders (replaces hard-coded Color.white.opacity(0.58))
• Added Settings Appearance segmented picker: System / Light / Dark (persisted via @AppStorage)
• Updated widget text to use adaptive Color.primary / Color.secondary

Verification (Revyl remote build + device)

• Remote build: success (cursor-r-0ac20a4e5baf45a88e98263575a09786-e66b40-20260628-053758)
• Opened Settings (unauthenticated) via bottom gear → confirmed Appearance section with System/Light/Dark picker
• Selected Dark → Settings switched to black background with light text; map chrome and bottom panel updated to dark styling

Before (light mode, Appearance picker visible)

After (dark mode selected in Settings)

After (map screen in dark mode)

Device verification result: Appearance picker renders on Settings (unauthenticated). Selecting Dark immediately applies dark styling to Settings (black grouped background, white primary text, Dark segment selected) and the main map UI (dark bottom sign-in panel).