Add line wrap toggle for editor and paste viewer (#130)

Shared client-side preference in localStorage 'palette-wrap', default
off. Toggle button on /new editor head and paste title bar, plus a Line
wrap checkbox in the settings menu; all read/write the same preference.
Active state uses the accent treatment like other toggles. No em dashes
in UI text.

internal/web/templates/new.html

@@ -16,6 +16,7 @@
           <option>markdown</option><option>text</option>
         </select>
         <button class="btn btn-icon" id="reguess" title="Re-detect language" type="button"><svg viewBox="0 0 24 24" fill="none" stroke="currentColor" stroke-width="2.4" stroke-linecap="round" stroke-linejoin="round" aria-hidden="true"><path d="M21 12a9 9 0 1 1-2.64-6.36"/><polyline points="21 3 21 9 15 9"/></svg></button>
+        <button type="button" class="btn btn-icon wrap-toggle" id="wrap-toggle" title="Toggle line wrap" aria-pressed="false">wrap</button>
       </div>
     </div>
     <div class="float editor-wrap">