feat: implement universal icon system with Phosphor Icons

- Install @phosphor-icons/react package
- Create centralized Icon component with size/weight/color variants
- Create icon registry with 34 icons across 5 categories
- Replace all raw Unicode symbols with proper icon components
- Add icons to navigation, buttons, status indicators, git operations
- Add icon CSS with consistent sizing and spacing
- Fix type definitions for Phosphor icon compatibility

Quality gates: typecheck ✓, lint ✓, build ✓ (375KB bundle)

apps/web/src/styles.css

@@ -1742,3 +1742,57 @@ pre[class*="language-"] {
 .token.variable {
   color: #ec4899;
 }
+
+/* Icon System */
+.icon {
+  display: inline-flex;
+  align-items: center;
+  justify-content: center;
+  flex-shrink: 0;
+  vertical-align: middle;
+}
+
+.icon svg {
+  display: block;
+}
+
+.icon-sm {
+  width: 16px;
+  height: 16px;
+}
+
+.icon-md {
+  width: 20px;
+  height: 20px;
+}
+
+.icon-lg {
+  width: 24px;
+  height: 24px;
+}
+
+.icon-xl {
+  width: 32px;
+  height: 32px;
+}
+
+/* Button icons */
+button .icon,
+a .icon {
+  margin-right: 0.35rem;
+}
+
+button .icon:last-child,
+a .icon:last-child {
+  margin-right: 0;
+}
+
+/* Navigation icons */
+.nav-item .icon {
+  margin-right: 0.5rem;
+}
+
+/* Status badge icons */
+.status-badge .icon {
+  margin-right: 0.25rem;
+}