[Web/PWA] Add settings surfaces for sync, model providers, embeddings, fonts, and local keys

[AlphaMao1]

Parent

#11

What to build

Build the first-version settings surface needed by the Web/PWA product: account/sync status, model providers, chat API keys, embedding provider configuration, font/reader preferences, and explicit local-only credential boundaries.

This issue should replace implicit demo settings with real configuration surfaces that later indexing and chat issues can depend on.

Acceptance criteria

• Users can see signed-in account/sync status and sign out from settings.
• Users can configure at least one chat provider such as DeepSeek or OpenAI-compatible providers.
• Users can configure at least one embedding provider such as Gemini Embedding or an OpenAI-compatible embedding endpoint.
• Provider API keys are stored only in browser-local storage or an equivalent local-only browser store and are never written to Supabase.
• Settings include reader preferences such as font size/default font/theme where they affect the reader.
• Settings show clear validation, connection-test, missing-key, invalid-key, and offline states.
• A network/storage inspection or automated test proves provider keys are not persisted to Supabase.
• The settings UI follows the approved visual system on desktop and Android/narrow layouts.

Blocked by

• [Web/PWA] Add Supabase auth and private data foundation #13
• [Web/PWA][UI] Apply selected visual system to app shell and core screens #23