[Web/PWA] Add skills library and slash command entry points for the reader assistant

[AlphaMao1]

Parent

#11

What to build

Restore the practical reader-assistant command surface from the current DeepReader release in a browser-safe way: a small skills library, visible skill enablement, and slash/quick-command entry points that feed the AI conversation without pretending to be a full desktop plugin system.

Acceptance criteria

• Users can view the first-version skills/commands available to the reader assistant.
• Users can enable/disable or at least understand which commands are active for the current book.
• The chat input supports command entry for core commands such as /记住, 总结本章, 分析观点, and 生成笔记 or equivalent localized commands.
• Commands route to the existing AI context path from [Web/PWA] Add AI chat sidebar and bottom sheet with reading context routing #18 and memory path from [Web/PWA] Add manual and confirmed AI memory saving #19 where relevant.
• Unsupported or unavailable commands show clear copy instead of silent no-ops.
• Skills/commands do not execute arbitrary local files, desktop plugins, or Tauri-only behavior.
• The skills surface works in desktop and Android/narrow layouts under the approved visual system.

Blocked by

• [Web/PWA] Add AI chat sidebar and bottom sheet with reading context routing #18
• [Web/PWA] Add manual and confirmed AI memory saving #19
• [Web/PWA][UI] Apply selected visual system to app shell and core screens #23