Add configurable tick interval (time between trades)

- Seed tick_interval_seconds setting (default 60s)
- Expose tick_interval_seconds via GET/PATCH /api/settings
- startScheduler reads the setting at start time; enforces 5s minimum
- getSchedulerStatus returns intervalSeconds for the UI
- Main page: "Every [__] s" input in idle bar — saves on blur, persists across
  restarts; running state displays "every Ns" next to symbol/action

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

lib/db.ts

@@ -161,6 +161,7 @@ db.exec(`
 // Seed defaults if missing
 const seedSetting = db.prepare(`INSERT OR IGNORE INTO settings (key, value) VALUES (?, ?)`);
 seedSetting.run('max_concurrent_accounts', '5');
+seedSetting.run('tick_interval_seconds', '60');
 
 export function getSetting(key: string): string | null {
     const row = db.prepare('SELECT value FROM settings WHERE key = ?').get(key) as { value: string } | undefined;