[Web/PWA][UI] Apply selected visual system to app shell and core screens

[AlphaMao1]

Parent

#11

What to build

Apply the approved visual direction to the Web/PWA app shell and core placeholder screens, replacing the current demo look with a coherent reader product UI. This issue is about visual system implementation and responsive layout quality only; it should not implement Supabase sync, EPUB parsing, RAG, or AI provider behavior.

Acceptance criteria

• The approved design direction from [Web/PWA][Design] Define DeepReader Web visual direction from the new design system #22 is applied to the Web/PWA shell.
• Library, reader, settings, notes/annotations, and AI surfaces share one consistent visual system.
• The desktop reader preserves a clear multi-panel reading workspace when space allows.
• The Android/narrow viewport prioritizes reading and does not cover core text with persistent AI or menu panels.
• All user-facing placeholder copy is Chinese and product-facing.
• No development scaffolding language, issue numbers, TODO labels, fake success claims, or no-Tauri badges appear in the UI.
• Visual QA screenshots are captured for desktop library, desktop reader, settings, and Android/narrow reader.
• Biome, typecheck, and build pass after the style refactor.

Blocked by

• [Web/PWA][Design] Define DeepReader Web visual direction from the new design system #22
• [Web/PWA] Scaffold reader app shell #12