Add contact-aware feed scope

- src/feed.rs: contact_feed + contact_pubkeys fetch the active profile's
  kind 3 contact list and aggregate notes only from those authors;
  aggregate_for + FeedBuilder accept an optional author whitelist
- IPC: FeedGet accepts contacts_only, resolves the active profile npub
- CLI: feed --contacts [limit] filters to the active profile's contacts
- Frontend: Feed screen Everyone/My contacts toggle, scope-aware empty
  states, feedGet(limit, contactsOnly) threading
- Tests for author filtering and the contacts scope in feed.rs and
  FeedScreen.test.tsx

frontend/src/styles.css

@@ -202,6 +202,56 @@ a {
   font-size: 14px;
 }
 
+.feed-actions {
+  display: flex;
+  align-items: center;
+  gap: 10px;
+  flex-wrap: wrap;
+}
+
+.segmented {
+  display: inline-flex;
+  align-items: center;
+  gap: 2px;
+  padding: 3px;
+  background: var(--surface-2);
+  border: 1px solid var(--border);
+  border-radius: 8px;
+}
+
+.segmented-btn {
+  display: inline-flex;
+  align-items: center;
+  gap: 6px;
+  padding: 5px 10px;
+  border: none;
+  border-radius: 6px;
+  background: transparent;
+  color: var(--text-muted);
+  font-size: 13px;
+  font-weight: 500;
+  cursor: pointer;
+  transition:
+    background 0.12s ease,
+    color 0.12s ease;
+}
+
+.segmented-btn:hover:not(:disabled):not(.is-active) {
+  background: var(--surface-hover);
+  color: var(--text);
+}
+
+.segmented-btn.is-active {
+  background: var(--surface);
+  color: var(--text);
+  box-shadow: 0 1px 2px rgba(0, 0, 0, 0.12);
+}
+
+.segmented-btn:disabled {
+  opacity: 0.45;
+  cursor: not-allowed;
+}
+
 /* -------------------------------------------------------------------------
    Sidebar
    ------------------------------------------------------------------------- */