Show server reception time before the message time

- Store a 'received' field (server clock) with every message
- New first column with the reception time
- Settings: time zone (browser, UTC, ~80 zones) and reception time
  format (YYYY-MM-DD - HH:MM:SS:mmm by default, ISO 8601, 12h, epoch...)
- The chosen time zone applies to every date shown

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

web/index.html

@@ -104,6 +104,18 @@
         </div>
       </section>
 
+      <section class="set-section">
+        <h3 data-i18n="dateTime">Date &amp; time</h3>
+        <div class="field-grid">
+          <label for="tzSelect" data-i18n="tzLabel">Time zone</label>
+          <select id="tzSelect" class="field"></select>
+          <label for="fmtSelect" data-i18n="fmtLabel">Reception time format</label>
+          <select id="fmtSelect" class="field"></select>
+        </div>
+        <p id="timePreview" class="time-preview"></p>
+        <p class="muted small" data-i18n="timeHelp"></p>
+      </section>
+
       <section class="set-section">
         <h3 data-i18n="tagsTitle">Color tags</h3>
         <p class="muted small" data-i18n="tagsHelp"></p>