security: require encryption for all sync channels, reduce activity log to 100

Sync operations (browser sync, Gist, custom URL, sync code) now refuse to
operate without encryption enabled. Disabling encryption also turns off all
active sync channels. Activity log cap reduced from 200 to 100 entries for
both storage and display.

https://claude.ai/code/session_01KF4i7Ra7zCEDskxDBaNtcT

src/options/options.html

@@ -92,7 +92,7 @@
           <label>Max log entries</label>
           <p class="setting-desc">Number of activity log entries to keep</p>
         </div>
-        <input type="number" id="maxLogEntries" class="input-small" min="10" max="1000" value="200">
+        <input type="number" id="maxLogEntries" class="input-small" min="10" max="1000" value="100">
       </div>
     </section>
 
@@ -106,7 +106,7 @@
           <span>&#128274;</span> Sync Encryption
         </h3>
         <p class="section-desc" style="margin-bottom:10px">
-          Encrypt your sync data with a password, TOTP, or both. Authentication is only required when new data arrives and your cached key has expired.
+          Encryption is required for all sync channels. Set up a password (and optionally TOTP) before enabling any sync method. Authentication is only required when new data arrives and your cached key has expired.
         </p>
 
         <div id="syncEncryptionSetup">