feat(ext/popup): field history view — masked values with reveal toggle

Shows current + historical values for tracked fields (password/concealed).
Click to reveal, copy button per entry (plaintext stored in a module-level
Map, never embedded in the DOM). Grouped by field name if multiple tracked
fields exist. Adds historyItemId to PopupState and 'field-history' to View.

Co-Authored-By: Claude <noreply@anthropic.com>

extension/src/popup/styles.css

@@ -1133,3 +1133,81 @@ textarea {
   opacity: 0.5;
   cursor: default;
 }
+
+/* --- Field history view --- */
+
+.history-header {
+  display: flex;
+  align-items: center;
+  gap: 12px;
+  margin-bottom: 8px;
+}
+
+.history-item-title {
+  font-size: 14px;
+  font-weight: 600;
+  color: #c9d1d9;
+  margin-bottom: 12px;
+}
+
+.history-field-label {
+  font-size: 11px;
+  color: #8b949e;
+  text-transform: uppercase;
+  margin: 12px 0 6px;
+}
+
+.history-entry {
+  display: flex;
+  align-items: center;
+  gap: 8px;
+  padding: 10px;
+  border-radius: 4px;
+  background: #161b22;
+  margin-bottom: 6px;
+  cursor: pointer;
+}
+
+.history-entry:hover {
+  background: #1c2128;
+}
+
+.history-entry__value {
+  flex: 1;
+  font-family: monospace;
+  font-size: 13px;
+}
+
+.history-entry__value.masked {
+  color: #8b949e;
+}
+
+.history-entry__value.revealed {
+  color: #c9d1d9;
+}
+
+.history-entry__meta {
+  display: flex;
+  flex-direction: column;
+  align-items: flex-end;
+  gap: 2px;
+  font-size: 11px;
+  color: #8b949e;
+}
+
+.history-entry__current {
+  color: #58a6ff;
+  font-weight: 500;
+}
+
+.history-entry__copy {
+  background: none;
+  border: none;
+  cursor: pointer;
+  font-size: 14px;
+  padding: 4px;
+}
+
+.history-entry__copy:hover {
+  opacity: 0.8;
+}