Thinner check lines; second signature line toggle

- Halve all check line thicknesses (line_thick * 0.5)
- Add second_signature column to account table with runtime migration
- Account settings toggle: enables a second signature line drawn 0.25"
  above the primary signature line, matching its width and thickness

public/index.html

@@ -346,6 +346,11 @@
           </div>
         </div>
 
+        <p class="settings-section-label">Signature</p>
+        <div class="form-group form-group-check">
+          <label><input type="checkbox" id="as-second-sig" name="second_signature"> Add second signature line (0.25&quot; above primary)</label>
+        </div>
+
         <p class="settings-section-label">Logo</p>
         <div class="form-group">
           <label for="as-logo">Upload new logo</label>