feat(chat): add copy, share, and edit action buttons above input bar

Add ChatActionBar component with three icon buttons above the chat
input:
- Copy: copies last assistant response to clipboard
- Share: copies full conversation text to clipboard
- Edit: removes last user message and places it back in input

Buttons are disabled when no relevant messages exist.

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

assets/main.css

@@ -1884,6 +1884,44 @@ h6 {
     color: var(--accent);
 }
 
+/* -- Chat Action Bar -- */
+.chat-action-bar {
+    display: flex;
+    align-items: center;
+    gap: 4px;
+    padding: 4px 24px 0;
+    background-color: var(--bg-sidebar);
+}
+
+.chat-action-btn {
+    display: inline-flex;
+    align-items: center;
+    gap: 5px;
+    padding: 4px 10px;
+    background: none;
+    border: 1px solid transparent;
+    border-radius: 6px;
+    color: var(--text-secondary);
+    font-size: 12px;
+    cursor: pointer;
+    transition: all 0.15s ease;
+}
+
+.chat-action-btn:hover:not(:disabled) {
+    color: var(--text-primary);
+    background-color: var(--bg-card);
+    border-color: var(--border-secondary);
+}
+
+.chat-action-btn:disabled {
+    opacity: 0.35;
+    cursor: not-allowed;
+}
+
+.chat-action-label {
+    font-family: 'Inter', sans-serif;
+}
+
 /* -- Chat Input Bar -- */
 .chat-input-bar {
     display: flex;