feat(feed): optional per-feed sender-in-title toggle

Add a per-feed senderInTitle flag (domain FeedState.senderInTitle ↔
FeedConfig.sender_in_title). When set, the feed generator prefixes each
entry title with [Sender] (display name, falling back to the address).
Exposed as an admin edit-form checkbox and across the REST API
create/update/response schemas.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/styles/components.css

@@ -295,6 +295,19 @@ label {
   color: var(--color-text-secondary);
 }
 
+.checkbox-label {
+  display: flex;
+  align-items: center;
+  gap: var(--spacing-sm);
+  cursor: pointer;
+}
+
+.checkbox-label input[type="checkbox"] {
+  width: auto;
+  margin: 0;
+  cursor: pointer;
+}
+
 input[type="text"],
 input[type="email"],
 input[type="search"],