fix(settings): add provider ordering controls

[Mingholy]

Summary

• Add sortable provider cards in Settings using @dnd-kit.
• Add visible reorder handles with PointerSensor and KeyboardSensor accessibility.
• Persist provider maps in the user-selected fallback priority order.

Before / After

Before (main)	After (fix/bf7-provider-ordering-a11y)
Provider settings without reorder handles	Provider settings after moving Anthropic before DeepSeek via the sortable handle

Test plan

• bun test web/test/SettingsPage.provider-order.test.ts
• cd web && bunx tsc -b
• bun run build
• Chrome DevTools: verified reorder handle keyboard flow with Space, ArrowUp, Space and captured before/after screenshots.
• Lint: repository has no lint script or ESLint/Biome/Oxlint config.