Fix push notifications: focus suppression, empty body, error visibility

- sw.js: suppress notification when the target chat tab is already focused
  (clients.matchAll visibility check before showNotification)
- generation_task.py: provide meaningful body for tool-only responses
  (lists tool names instead of sending an empty string that browsers discard);
  promote scheduling failure from debug to warning
- push.py: promote send errors from warning to error with exc_info;
  log successful sends at INFO so they're visible in normal operation

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

src/fabledassistant/services/generation_task.py

@@ -358,9 +358,18 @@ async def run_generation(
         try:
             from fabledassistant.services.push import send_push_notification, vapid_enabled
             if vapid_enabled():
-                preview = buf.content_so_far[:120].rstrip()
-                if len(buf.content_so_far) > 120:
-                    preview += "…"
+                text = buf.content_so_far.strip()
+                if text:
+                    preview = text[:120].rstrip()
+                    if len(text) > 120:
+                        preview += "…"
+                else:
+                    # Tool-only response — summarise what was done
+                    tool_names = [tc.get("function") for tc in all_tool_calls if tc.get("function")]
+                    if tool_names:
+                        preview = f"Completed: {', '.join(tool_names[:3])}"
+                    else:
+                        preview = "Action completed"
                 asyncio.create_task(send_push_notification(
                     user_id,
                     title="Response ready",
@@ -368,7 +377,7 @@ async def run_generation(
                     url=f"/chat/{conv_id}",
                 ))
         except Exception:
-            logger.debug("Failed to schedule push notification", exc_info=True)
+            logger.warning("Failed to schedule push notification", exc_info=True)
 
         # Title generation is non-critical — fire-and-forget so done fires immediately
         non_system = [m for m in messages if m["role"] != "system"]