feat: add immersive reading workspace

[zha-chicken]

Summary

• add an Immersive Reading workspace for TXT, PDF, EPUB, MOBI, FB2, XPS, and Markdown/text books
• track chapter progress, citations, selection actions, and source-faithful chat references
• gate chapter transitions with Focus-Checks while skipping front matter and preserving passed checkpoints
• add exact, fuzzy, fast-description, and fine-description search modes

Description search

• builds persistent per-chapter retrieval cards in the background with bounded concurrency
• routes fast queries without extended reasoning, then deep-searches candidate chapters in parallel
• automatically falls back to the existing fine full-book search when confidence is low
• runs long description searches as background jobs polled by the UI to avoid proxy timeouts
• enables description matching only when the configured default model exposes at least a 50k context window

Focus-Check behavior

• evaluates only the current checkpoint chapter, or the current 20k-character chunk for unchaptered books
• skips front matter and very short prefaces
• preserves passed checkpoints until the reader chooses "Read Immersively Again"
• uses the default model without extended reasoning and allows up to 255k output tokens
• surfaces model/runtime failures as transient notifications instead of converting them into zero scores

Chat integration

• lets readers attach selected chapters from imported books to regular chat
• adds the selected reading sections to the source inventory and runtime context

Validation

• pytest tests/immersive_reading/test_service.py tests/services/session/test_source_inventory.py tests/services/session/test_regenerate.py tests/api/test_unified_ws_turn_runtime.py -q --disable-warnings
• ruff check on all changed Python files
• ruff format --check on all changed Python files
• npx tsc --noEmit
• npm run lint -- --quiet
• npm run i18n:check
• npm run build
• git diff --check

[zha-chicken]

Immersive Reading UI screenshots: fast description search, Focus-Check, and text-selection actions.