feat(contacts): name-based blocking, fix CSS breakpoint

- New blocked_names table for blocking bots without known public_key
- get_blocked_contact_names() returns union of pubkey-blocked + name-blocked
- POST /api/contacts/block-name endpoint for name-based blocking
- GET /api/contacts/blocked-names-list for management UI
- Block button always visible in chat (falls back to name-based block)
- Blocked Names section shown in Existing Contacts Blocked filter
- CSS breakpoint for icon-only buttons: 768px → 428px (iPhone-sized)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

app/static/css/style.css

@@ -333,18 +333,6 @@ main {
         font-size: 0.875rem;
     }
 
-    /* Contact card buttons: icon-only on mobile */
-    .pending-contact-card .btn-label,
-    .existing-contact-card .btn-label {
-        display: none;
-    }
-
-    /* Allow button wrapping on small screens */
-    .pending-contact-card .d-flex.gap-2,
-    .existing-contact-card .d-flex.gap-2 {
-        flex-wrap: wrap;
-    }
-
     /* Modal: Better mobile layout */
     .modal-dialog {
         margin: 0.5rem;
@@ -360,6 +348,19 @@ main {
     }
 }
 
+/* Small phone screens: icon-only buttons in contact cards */
+@media (max-width: 428px) {
+    .pending-contact-card .btn-label,
+    .existing-contact-card .btn-label {
+        display: none;
+    }
+
+    .pending-contact-card .d-flex.gap-2,
+    .existing-contact-card .d-flex.gap-2 {
+        flex-wrap: wrap;
+    }
+}
+
 /* Loading State */
 .loading-spinner {
     display: flex;