feat(ui): add shared Spinner primitive and replace one-off loading spinners

[zainabwahab-eth]

Closes #403

Changes

• New component: src/components/ui/Spinner.tsx - A shared, accessible Spinner primitive with size variants (sm/md/lg) and label prop for screen reader announcements
• Pools (src/pages/Pools.tsx): Replaced one-off inline SVG spinner with shared Spinner (size="lg", label="Loading pools")
• Profile (src/pages/Profile.tsx): Replaced one-off Loader2 spinner with shared Spinner (size="lg", label="Loading profile")
• Connect (src/pages/Connect.tsx): Replaced two one-off Loader2 spinners with shared Spinner (size="sm", labels "Validating address" and default)

Accessibility

All three call sites now use the shared Spinner which includes:

• role="status" ARIA attribute
• aria-label prop passed through from each call site so screen readers announce the loading state
• motion-safe:animate-spin for reduced motion preference

Validation

• Type-check: passed
• Lint: 6 pre-existing errors (unrelated to this change)
• Tests: 780/780 passed
• Build: passed
• Accessibility smoke test: 4/4 passed

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