fix: persist preferences to localStorage

[interview35]

Summary

The Preferences page at /settings/preferences showed a "Preferences saved locally (mock)" toast when clicking Save, but never actually persisted anything. All settings (density, table row height, line numbers, notifications, auto-save) were lost on every page refresh.

What this PR does

• Consolidates the five separate useState calls into a single PreferencesData object managed by one useState
• Adds loadPreferences() which reads from localStorage under the key faros.preferences, with a try-catch fallback to defaults if the stored data is corrupted
• Initializes state from loadPreferences() so preferences survive page refreshes
• Updates handleSave to serialize the preferences object to localStorage before showing the toast
• Removes "(mock)" from the toast message since it now actually saves
• Adds a typed updatePref helper to keep the setter calls clean and type-safe

File changed

frontend/src/pages/Settings/Preferences.tsx — no other files touched.

Verification

• Reviewed the diff carefully to confirm all five preference controls (density, tableRowSize, showLineNumbers, enableNotifications, autoSaveDrafts) read from and write to the unified prefs state
• Confirmed loadPreferences merges with defaults so adding new preference fields later won't crash on old stored data
• Confirmed the handleSave try-catch handles localStorage being unavailable (private browsing, quota exceeded)
• The theme preference is intentionally left to useTheme's own persistence mechanism — this PR only covers the five preferences that had no persistence at all