feat: React + FastAPI web UI to replace NiceGUI

[biswasbiplob]

Summary

• Replace NiceGUI with a modern React 19 + TypeScript + Tailwind CSS v4 frontend backed by a FastAPI REST API with SSE progress streaming
• Complete implementation of the migration plan in docs/UIUpgradePlan.md
• Version bumped to 0.8.0

Backend (src/legacylipi/api/)

• FastAPI app with session-based architecture and async pipeline runners
• 11 REST endpoints: config (3), sessions (2), processing (3), progress SSE (1), download (1), health (1)
• Pydantic request/response schemas matching all frontend contracts
• In-memory session manager with TTL cleanup
• Pipeline logic extracted from NiceGUI app.py into standalone async functions

Frontend (frontend/)

• 10 React components: Header, FileUploader (drag-drop), WorkflowModeSelector, ScanCopySettings, OcrSettings, OutputFormatSelect, TranslationSettings, TranslatorSettings, ActionButton, StatusPanel
• 5 custom hooks: useFileUpload, useProgress (SSE), useProcessing, useDownload, useConfig
• React Context + useReducer state management with 11 action types
• Typed API client with fetch wrapper
• Dark theme with CSS variables, responsive 3-column layout

Integration

• legacylipi api CLI subcommand + legacylipi-web entry point
• scripts/dev.sh for hot-reload development (FastAPI + Vite HMR)
• scripts/check.sh updated with frontend TypeScript check (5 steps)
• FastAPI serves built React static files in production

Tests & Docs

• 14 new API tests (393 total, all passing)
• Updated README.md, docs/development.md, CLAUDE.md with new architecture

Test plan

• tsc --noEmit passes (zero TypeScript errors)
• vite build produces 227KB JS + 20KB CSS
• ruff format --check + ruff check pass
• mypy passes with no issues
• pytest — 393 tests pass (14 new API tests)
• Manual: uv run legacylipi api serves UI at :8000
• Manual: Upload PDF → select workflow → process → download result

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