feat: self-hosted favicon resolver via /favicon/<domain>

Adds a Kafka-hosted favicon proxy at /favicon/<domain>:
- Fetches favicon.ico from the target domain
- In-memory cache with 1-hour TTL and ETag support (304 Not Modified)
- Max 64KB per favicon to prevent memory abuse
- Privacy: user browser talks to Kafka, not Google/DuckDuckGo

New "Self (Kafka)" option in the favicon service selector.
Defaults to None. No third-party requests when self is chosen.

internal/views/static/js/settings.js

@@ -48,7 +48,8 @@ function applyTheme(theme) {
 function applyFavicon(service) {
   var faviconMap = {
     google: function(domain) { return 'https://www.google.com/s2/favicons?domain=' + encodeURIComponent(domain) + '&sz=32'; },
-    duckduckgo: function(domain) { return 'https://icons.duckduckgo.com/ip3/' + encodeURIComponent(domain) + '.ico'; }
+    duckduckgo: function(domain) { return 'https://icons.duckduckgo.com/ip3/' + encodeURIComponent(domain) + '.ico'; },
+    self: function(domain) { return '/favicon/' + encodeURIComponent(domain); }
   };
   var imgs = document.querySelectorAll('.result-favicon');
   imgs.forEach(function(img) {
@@ -56,7 +57,7 @@ function applyFavicon(service) {
     if (!domain) return;
     if (service === 'none') {
       img.style.display = 'none';
-    } else {
+    } else if (faviconMap[service]) {
       img.style.display = '';
       img.src = faviconMap[service](domain);
     }
@@ -125,8 +126,8 @@ function renderPanel(prefs) {
 
   
   var faviconOptions = '';
-  ['none', 'google', 'duckduckgo'].forEach(function(src) {
-    var labels = { none: 'None', google: 'Google', duckduckgo: 'DuckDuckGo' };
+  ['none', 'google', 'duckduckgo', 'self'].forEach(function(src) {
+    var labels = { none: 'None', google: 'Google', duckduckgo: 'DuckDuckGo', self: 'Self (Kafka)' };
     var selected = prefs.favicon === src ? ' selected' : '';
     faviconOptions += '<option value="' + src + '"' + selected + '>' + labels[src] + '</option>';
   });

internal/views/templates/preferences.html

@@ -78,6 +78,7 @@
           <option value="none" selected>None</option>
           <option value="google">Google</option>
           <option value="duckduckgo">DuckDuckGo</option>
+          <option value="self">Self (Kafka)</option>
         </select>
       </div>
     </section>