fix(session): clear persisted theme preference on logout and sync STORAGE_KEYS

[rudra496]

Closes #8

Summary of Changes

1. Synchronized STORAGE_KEYS:

   • Added THEME_PREFERENCE: 'mindmint_theme_preference' to STORAGE_KEYS in src/session/storageKeys.ts.
   • Updated src/theme/themeStore.ts to reference STORAGE_KEYS.THEME_PREFERENCE for Zustand persistence instead of a hardcoded string.

2. Zustand Theme Store Reset & Storage Clearing:

   • Exposed
     esetTheme action in useThemeStore to revert preference back to DEFAULT_THEME_PREFERENCE ('system').
   • Integrated useThemeStore.getState().resetTheme() and useThemeStore.persist.clearStorage() into clearSession() in src/session/clearSession.ts to ensure complete session artifact cleanup across logouts.

3. Testing & Verification:

   • Added unit test in src/session/clearSession.test.ts verifying that clearSession() resets the theme store and deletes mindmint_theme_preference from localStorage.
   • Verified 100% test pass rate (
     pm test), linting (
     pm run lint), and production build (
     pm run build).

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[rudra496]

Same situation as #26: the red check comes from main's broken package-lock.json (#24/#25 left it missing several entries), not from this fix. See #30 for the minimal repair — once it lands, this check should go green.