Harden JSON storage (atomic writes + crash-safe loads) + suggestions lightbulb

- storage.py: atomic save_json (temp+fsync+os.replace) and load_json that
  survives missing/corrupt files (moves corrupt aside). Closes #11/#12.
- route all data modules + app.py helpers through storage (closes #31 core);
  fixes mutable DEFAULT_FUNDS/DEFAULT_CARS return (#23).
- gunicorn --workers 1 --threads 4 to remove the write race (#13); strip
  Dockerfile template scaffolding (#40).
- base.html: 💡 suggestions link in the shared nav (shows on every page).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

templates/base.html

@@ -23,6 +23,7 @@
       <a href="{{ url_for('debts_view') }}">Debt</a>
       <a href="{{ url_for('net_worth_view') }}">Net Worth</a>
       <a href="{{ url_for('summary') }}">Spending Report</a>
+      <a href="{{ url_for('suggestions.suggestions_page') }}" class="nav-suggest" title="Suggestions &amp; feedback" aria-label="Suggestions">💡</a>
     </div>
   </nav>
   <main>