docs: initialize project documentation

Adds the standard ./docs/ structure (overview, codebase summary,
architecture, code standards, design guidelines, deployment guide,
roadmap) and the code-review report under ./plans/reports/.
README now points at the docs and covers the codeserver dev profile.

web/docs/development-roadmap.md

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+# Development Roadmap
+
+This document tracks **future work only**. Completed features live in git commit history, not here.
+
+## Currently Implemented Features
+
+The app is fully functional for core gameplay:
+- 9×9 player card generation with weighted number distribution
+- Cell marking (toggle crossed state)
+- Bingo detection and celebration popup
+- "Chờ X" waiting notifications
+- Host number drawing from 1–90 deck
+- 9×10 master board tracking called numbers
+- Host's own player card (isolated instance)
+- localStorage persistence
+- Dark mode
+- Mobile responsive
+- Offline capable
+
+## Idea Phase
+
+### Sound Effects on Bingo
+Play celebratory chime or "Kinh!" voice snippet when row completes. Could use Web Audio API or `<audio>` tag. **Status**: Idea (no demand yet)
+
+### Undo Last Cell
+Allow player to undo the most recent cross/uncross action. Requires change history or state snapshot. **Status**: Idea (low priority)
+
+### Theme Switcher
+Explicit light/dark toggle button instead of relying on OS preference. **Status**: Idea (Tailwind already supports OS toggle)
+
+### PWA Install
+Add service worker and manifest for "Install App" prompt on Android/iOS. **Status**: Idea (would require server-side components)
+
+## Considered Phase
+
+### Multiplayer Sync (Real-time)
+Host and players connect via WebSocket to sync called numbers and state. Requires backend server. Would enable:
+- Decoupled devices (players' phones, host's laptop on big screen)
+- Remote tournaments
+- Master board auto-updates all players in real-time
+
+**Consideration**: Out of scope for static export. Requires major refactor (Next.js API routes + WebSocket server). Deferred indefinitely.
+
+### i18n Beyond Vietnamese
+Internationalization (English, Chinese, etc.). Requires extraction of all Vietnamese strings and i18n library (next-intl). **Status**: Considered (low demand for non-Vietnamese users)
+
+### Undo/Redo System
+Full undo/redo stack with history navigation. Adds complexity to state management. **Status**: Considered (YAGNI for now)
+
+## Testing (Planned but Unstarted)
+
+### Unit Tests
+- Game logic: `generateGrid()`, `isRowComplete()`, `getWaitingNumber()`
+- localStorage helpers: `saveGrid()`, `loadGrid()`, etc.
+
+**Tech**: Jest + @testing-library/react
+
+### Component Tests
+- PlayerBoard with mocked localStorage
+- Master page with different game states
+
+### E2E Tests
+- Player flow: generate card → click cells → verify bingo popup
+- Host flow: new game → draw numbers → verify board state
+
+**Tech**: Playwright or Cypress
+
+## Future Enhancements (Speculative)
+
+### Export/Import Card
+Allow player to export their grid as image or JSON, import someone else's card. Use Canvas API or print-friendly CSS.
+
+### Leaderboard / Stats
+Track games won, time per bingo, etc. Requires server-side persistence. Out of scope for static export.
+
+### Accessibility Improvements
+- ARIA labels for grid cells
+- Keyboard navigation (arrow keys to move, Enter to toggle)
+- Screen reader support
+
+### Custom Number Range
+Host selects range (e.g., 1–75 for American bingo) instead of hardcoded 1–90. Requires config UI and refactor of game logic constants.
+
+### Different Grid Sizes
+Support 8×8 or 10×10 grids. Major refactor (NUM_ROWS, NUM_COLS constants, weighted selection algorithm).
+
+---
+
+## Decision Rationale
+
+All decisions follow **YAGNI** (You Aren't Gonna Need It):
+- No multiplayer sync → adds server dependency, breaks static export model
+- No i18n → Vietnamese-only community, localizing adds complexity without demand
+- No testing → small codebase, manual testing covers critical paths; add tests when code grows or bugs surface
+- No undo/redo → simple game, mistakes are part of play experience
+
+Future work gates on **real user demand**, not speculation.
+
+Last reviewed: 2026-04-26