Replace threshold placeholder link with a real settings panel

Adds a "Loss thresholds" panel to the popup backed by the backend's
new GET/PATCH /me/settings endpoints, converting between the wire's
fractional percentages and whole-number form inputs only at that
boundary.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01RTorFQXMR9ZQtCKcMTXLHp

src/entrypoints/popup/style.css

@@ -100,11 +100,65 @@ header .subtitle {
   background: rgba(128, 128, 128, 0.12);
 }
 
-.thresholds-link {
+.thresholds-toggle {
   display: block;
+  width: 100%;
   margin-top: 14px;
+  padding: 0;
+  border: none;
+  background: none;
   color: inherit;
-  opacity: 0.5;
-  text-decoration: none;
-  pointer-events: none;
+  opacity: 0.75;
+  font: inherit;
+  text-align: left;
+  cursor: pointer;
+}
+
+.thresholds-toggle:hover {
+  opacity: 1;
+}
+
+.thresholds-panel {
+  margin-top: 14px;
+  padding-top: 12px;
+  border-top: 1px solid rgba(128, 128, 128, 0.25);
+}
+
+.thresholds-panel h2 {
+  margin: 0 0 8px;
+  font-size: 13px;
+}
+
+.thresholds-panel label {
+  display: flex;
+  align-items: center;
+  justify-content: space-between;
+  gap: 8px;
+  margin-bottom: 8px;
+  font-size: 12px;
+  opacity: 0.85;
+}
+
+.threshold-input {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  flex-shrink: 0;
+}
+
+.threshold-input input {
+  width: 48px;
+  padding: 3px 5px;
+  border: 1px solid rgba(128, 128, 128, 0.4);
+  border-radius: 4px;
+  background: transparent;
+  color: inherit;
+  font: inherit;
+  text-align: right;
+}
+
+.thresholds-error {
+  margin: 4px 0 8px;
+  color: #dc2626;
+  font-size: 12px;
 }