Add loading state and toast feedback to Settings page save button

[Prasiejames]

Description

The Settings page has a "Save Settings" button but it does not show a loading spinner or disable itself while saving. Users might click it multiple times.

Requirements

• Add a loading state to the "Save Settings" button using the existing useAsyncAction hook pattern
• When loading, show a spinner and disable the button
• Show a success toast when settings are saved
• Show an error toast if saving fails

Relevant files

• src/pages/settings.tsx
• src/lib/errors/toast.tsx (for toast integration)

Complexity: Trivial (100 pts)

Good first issue. Small focused change with clear before/after behavior.