Sessions: edit form, soft-delete, lazy creation, chat as separate page

- Session click opens edit form (name, description, image, agent) instead of chat
- Chat is now a separate route (#chat/{id}), opened via button
- Sessions created on save, not on click
- Soft-delete for sessions (is_deleted flag)
- Delete button in session form
- Removed title double-click editing from chat
- Card images now square (aspect-ratio: 1)
- New Session as placeholder card

wwwroot/css/base.css

@@ -107,8 +107,7 @@ header {
 }
 
 .msg {
-  max-width: 85%;
-  padding: var(--s4);
+  padding: var(--s3) var(--s4);
   white-space: pre-wrap;
   word-break: break-word;
 }
@@ -129,6 +128,7 @@ header {
   align-self: center;
   border: var(--border-width) solid var(--f2-border);
   text-align: center;
+  padding: var(--s2);
 }
 
 /* ── Cursor ── */