feat(privacy): replace Google Favicons with local letter icons

Remove getFaviconUrl() and all external network requests. Bookmarks now
show a colored letter icon with deterministic hue based on title.
Eliminates privacy leak and Brave Shields compatibility issues.

src/css/main.css

@@ -562,12 +562,13 @@ html, body {
 
 body.compact .bm-item { padding: var(--spacing-compact) 10px; }
 
-.bm-favicon { width: 14px; height: 14px; flex-shrink: 0; border-radius: 2px; opacity: 0.85; }
-.bm-favicon-fallback {
-  width: 14px; height: 14px; flex-shrink: 0;
-  background: var(--accent-dim); border-radius: 2px;
+.bm-favicon-local {
+  width: 16px; height: 16px; flex-shrink: 0;
+  border-radius: 3px;
   display: flex; align-items: center; justify-content: center;
-  font-size: 8px; color: var(--accent);
+  font-size: 9px; font-weight: 600;
+  color: #fff;
+  line-height: 1;
 }
 .bm-text { flex: 1; min-width: 0; }
 .bm-title { font-size: 12px; font-weight: 400; color: var(--text-primary); line-height: 1.3; }

src/js/boards.js

@@ -215,19 +215,11 @@ function createBmEl(bm) {
   li.dataset.bmUrl = bm.url;
   li.draggable = true;
 
-  const favicon = document.createElement('img');
-  favicon.className = 'bm-favicon';
-  favicon.width = 14;
-  favicon.height = 14;
-  favicon.src = getFaviconUrl(bm.url);
-  favicon.addEventListener('error', function() {
-    this.classList.add('hidden');
-    this.nextElementSibling.classList.remove('hidden');
-  });
-
-  const fallback = document.createElement('div');
-  fallback.className = 'bm-favicon-fallback hidden';
-  fallback.textContent = bm.title.charAt(0).toUpperCase();
+  const favicon = document.createElement('div');
+  favicon.className = 'bm-favicon-local';
+  favicon.textContent = bm.title.charAt(0).toUpperCase();
+  const hue = (bm.title.charCodeAt(0) * 137) % 360;
+  favicon.style.backgroundColor = `hsl(${hue}, 45%, 35%)`;
 
   const textDiv = document.createElement('div');
   textDiv.className = 'bm-text';
@@ -247,7 +239,6 @@ function createBmEl(bm) {
   deleteBtn.textContent = '✕';
 
   li.appendChild(favicon);
-  li.appendChild(fallback);
   li.appendChild(textDiv);
   li.appendChild(deleteBtn);
 

src/js/state.js

@@ -33,15 +33,6 @@ function escHtml(str) {
     .replace(/"/g, '"');
 }
 
-function getFaviconUrl(url) {
-  try {
-    const u = new URL(url);
-    return `https://www.google.com/s2/favicons?domain=${u.hostname}&sz=16`;
-  } catch {
-    return '';
-  }
-}
-
 function getDefaultBoards() {
   return [
     {