feat(tui): add terminal UI layer with ratatui

[Xaxxoo]

Summary

• Adds ratatui and crossterm as native-only dependencies in Cargo.toml
• Creates tui module (src/tui/mod.rs) with full TUI rendering components:
  • PuzzleView with bordered panels showing title, description, difficulty badge, and remaining hints
  • TimerBar countdown timer as a progress bar (Gauge widget)
  • ScoreDisplay with real-time score updates and colour coding
  • FooterBar with persistent keyboard shortcuts (h=Hint, Enter=Submit, q=Quit, ?=Help)
• Colour coding: green for solved, red for failed, yellow for in-progress
• TuiApp state management struct with tick-based timer, auto-fail on expiry
• --no-tui CLI flag preserves the existing CLI module for headless environments
• TUI module gated behind #[cfg(not(feature = "wasm"))] for WASM compatibility
• Fixes pre-existing build errors (duplicate module declarations, missing time module, clippy warnings)

Test plan

• 20 unit tests covering: game status colour mapping, puzzle state transitions, timer state (new, progress ratio, zero total, tick, expiry), score state colours, difficulty labels/colours, shortcut formatting, puzzle view data construction, TuiApp lifecycle (new, set puzzle, update score, mark solved/failed, tick causes failure, tick preserves solved)
• All 206 tests pass (cargo test)
• cargo clippy -- -D warnings passes clean
• cargo fmt --check passes clean

Closes #41

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