feat: session delete + touch-friendly message controls

Session delete:
- DELETE /sessions/{session_id} endpoint (chat.py + session_store.py)
- × button on each session item in the panel (hover-reveal on desktop)
- Clears UI if the active session is deleted

Touch accessibility:
- @media (hover: none) rule makes msg-actions always visible on touch devices
- msg-act-btn tap targets enlarged to 36px min-height, readable font size
- session-delete-btn also always visible and finger-sized on touch

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

cortex/static/style.css

@@ -176,6 +176,23 @@
         .session-item:hover { background: var(--bg); }
         .session-item.new { color: var(--accent); justify-content: center; }
 
+        .session-delete-btn {
+            background: none;
+            border: none;
+            color: var(--muted);
+            font-size: 1.1rem;
+            line-height: 1;
+            padding: 2px 4px;
+            cursor: pointer;
+            border-radius: 3px;
+            flex-shrink: 0;
+            margin-left: 4px;
+            opacity: 0;
+            transition: opacity 0.15s, color 0.15s;
+        }
+        .session-item:hover .session-delete-btn { opacity: 1; }
+        .session-delete-btn:hover { color: #e06c75; }
+
         .session-id {
             font-family: monospace;
             font-size: 0.85rem;
@@ -1029,6 +1046,29 @@
             #note-type-btn { padding: 6px 10px; }
         }
 
+        /* ── Touch devices — no hover capability ─────────────────── */
+        /* Always show message controls; make tap targets finger-sized */
+        @media (hover: none) {
+            .msg-actions {
+                opacity: 1;
+                padding: 4px 2px 2px;
+            }
+
+            .msg-act-btn {
+                font-size: 0.78rem;
+                padding: 6px 12px;
+                min-height: 36px;
+            }
+
+            .session-delete-btn {
+                opacity: 1;
+                font-size: 1.3rem;
+                padding: 4px 8px;
+                min-width: 36px;
+                min-height: 36px;
+            }
+        }
+
         @media (max-width: 380px) {
             header .name { font-size: 1rem; }
             .header-emoji { font-size: 1.3rem; }