feat: persist settings and scores in SQLite database

[manuelmauro]

Summary

Game settings and score history are now persisted in a SQLite database using rusqlite (bundled) and dirs for platform-appropriate storage paths.

Database location

Platform	Path
macOS	~/Library/Application Support/nback/nback.db
Linux	~/.local/share/nback/nback.db
Windows	C:\Users\<user>\AppData\Roaming\nback\nback.db

Falls back to ./nback/nback.db if no data directory is available.

Schema

-- Single-row settings (auto-created with defaults on first run)
CREATE TABLE settings (
    id, n, rounds, round_time, position, color, shape, sound
);

-- One row per completed game session
CREATE TABLE scores (
    id, played_at, n, total_rounds, round_duration,
    correct, wrong, f1_score_percent
);

Behavior

• Startup: database is opened/created, settings and last 50 scores loaded
• Settings change: PersistencePlugin watches GameSettings for changes and saves to DB in PostUpdate (covers both menu edits and end-of-game difficulty adjustments)
• Game end: score is inserted into DB, full history reloaded (so played_at timestamps are populated by SQLite)
• ScoreRecord gains a played_at: String field (ISO-8601, set by datetime('now') default)

Dependencies added

• rusqlite 0.35 with bundled feature (statically links SQLite)
• dirs 6.0 for platform data directories