fix(settings): persist API key, provider, model, and integrations

[NewCoder3294]

Summary

• The settings page was a static mockup — every input was a presentational component with no state, no onChange, and no persistence. A user pasting an Anthropic key into API_KEY.SAV saw it sit in the field briefly and vanish on reload.
• Adds a zustand+persist store at web/src/lib/settings-store.ts that caches provider, apiKey, model, and the three integration fields to localStorage under prometheus-settings-v1.
• Rewires web/src/app/settings/page.tsx to be controlled (provider buttons toggle the store, inputs are value/onChange-bound), with a hydration guard so values don't flash empty after reload, plus a new "Reset all" affordance.

Scope

• UI-only. Nothing leaves the browser. The Go API still reads its provider key from PROMETHEUS_ORCHESTRATOR_API_KEY at server boot — this store is the UI's working copy until a server-side settings endpoint exists.
• No backend changes. No new dependencies (zustand was already in web/package.json).
• Hydration is handled via the standard Next.js useEffect-on-mount pattern; SSR renders empty inputs and localStorage rehydrates client-side.

Test plan

• cd web && npm install && npm run dev — settings page loads.
• Paste a key into API_KEY.SAV, refresh — value persists.
• Click a different provider — selection persists across reloads.
• Click "Reset all" — all fields clear.
• Open in a private window — fields start blank (per-browser cache).

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