fix(sidepanel.html): add CRITICAL severity filter button, tag styles, card border color

extension/sidepanel.html

@@ -29,6 +29,7 @@
     .filter-btn.active { background: #18181b; color: #fff; border-color: #18181b; }
     .filter-count { font-size: 10px; opacity: 0.6; }
     .dot { width: 6px; height: 6px; border-radius: 50%; }
+    .dot-purple { background: #a855f7; }
     .dot-red { background: #ef4444; }
     .dot-amber { background: #f59e0b; }
     .dot-blue { background: #3b82f6; }
@@ -36,6 +37,7 @@
     .clause-list { padding: 8px; }
     .clause-card { border: 1px solid #e4e4e7; border-radius: 10px; padding: 12px; margin-bottom: 6px; transition: all 0.15s; cursor: default; }
     .clause-card:hover { border-color: #d4d4d8; box-shadow: 0 1px 3px rgba(0,0,0,0.04); }
+    .clause-card.sev-critical { border-left: 3px solid #a855f7; }
     .clause-card.sev-high { border-left: 3px solid #ef4444; }
     .clause-card.sev-medium { border-left: 3px solid #f59e0b; }
     .clause-card.sev-low { border-left: 3px solid #3b82f6; }
@@ -43,6 +45,7 @@
     .clause-tags { display: flex; flex-wrap: wrap; gap: 4px; }
     .tag { font-size: 10px; font-weight: 600; padding: 2px 8px; border-radius: 4px; border: 1px solid; display: inline-flex; align-items: center; gap: 3px; }
     .tag svg { width: 10px; height: 10px; }
+    .tag-critical { background: #faf5ff; color: #7c3aed; border-color: #d8b4fe; }
     .tag-high { background: #fef2f2; color: #b91c1c; border-color: #fecaca; }
     .tag-medium { background: #fffbeb; color: #a16207; border-color: #fde68a; }
     .tag-low { background: #eff6ff; color: #1d4ed8; border-color: #bfdbfe; }
@@ -71,6 +74,7 @@
 
   <div class="filters" id="filters" style="display:none;">
     <button class="filter-btn active" data-filter="all">All</button>
+    <button class="filter-btn" data-filter="CRITICAL" id="filter-critical" style="display:none;"><span class="dot dot-purple"></span>Critical <span class="filter-count" id="fc-crit">0</span></button>
     <button class="filter-btn" data-filter="HIGH"><span class="dot dot-red"></span>High <span class="filter-count" id="fc-high">0</span></button>
     <button class="filter-btn" data-filter="MEDIUM"><span class="dot dot-amber"></span>Medium <span class="filter-count" id="fc-med">0</span></button>
     <button class="filter-btn" data-filter="LOW"><span class="dot dot-blue"></span>Low <span class="filter-count" id="fc-low">0</span></button>