feat: multiple identity profiles

Identity tab now supports multiple named profiles:
- Dropdown selector to switch between profiles
- "+" button to add a new profile (prompts for name)
- Pencil button to rename
- X button to delete (can't delete the last one)
- Toggle to enable/disable each profile independently

Default profile is "Personal". All active profiles are merged
and substituted simultaneously — so "Personal" (your name) and
"Work" (your work email, company domain) both get caught.

Storage model: profiles are stored as an array under ss_identity.
getIdentity() merges all active profiles into a single identity
object for the substitution engine.

https://claude.ai/code/session_01Dvgwe7XMoSxnWXkih8p1Cw

src/popup/popup.css

@@ -276,6 +276,28 @@ body {
   font-weight: 600;
 }
 
+/* Profile bar */
+.profile-bar {
+  display: flex;
+  align-items: center;
+  gap: 4px;
+  margin-bottom: 8px;
+}
+
+.profile-bar .select {
+  font-weight: 500;
+}
+
+.profile-bar .btn-icon {
+  width: 28px;
+  height: 28px;
+  font-size: 16px;
+  display: flex;
+  align-items: center;
+  justify-content: center;
+  padding: 0;
+}
+
 /* Identity tab */
 .id-section {
   margin-bottom: 12px;