Handle non-JSON API responses gracefully

Some boorus return empty/HTML responses for tag-limited queries.
All API clients now catch JSON parse errors and return empty
results instead of crashing.

booru_viewer/core/api/danbooru.py

@@ -29,7 +29,10 @@ class DanbooruClient(BooruClient):
         if resp.status_code != 200:
             log.warning(f"  body: {resp.text[:500]}")
         resp.raise_for_status()
-        data = resp.json()
+        try:
+            data = resp.json()
+        except Exception:
+            return []
 
         # Some Danbooru forks wrap in {"posts": [...]}
         if isinstance(data, dict):

booru_viewer/core/api/e621.py

@@ -49,7 +49,10 @@ class E621Client(BooruClient):
         if resp.status_code != 200:
             log.warning(f"  body: {resp.text[:500]}")
         resp.raise_for_status()
-        data = resp.json()
+        try:
+            data = resp.json()
+        except Exception:
+            return []
 
         # e621 wraps posts in {"posts": [...]}
         if isinstance(data, dict):

booru_viewer/core/api/gelbooru.py

@@ -44,7 +44,11 @@ class GelbooruClient(BooruClient):
             log.warning(f"  body: {resp.text[:500]}")
         resp.raise_for_status()
 
-        data = resp.json()
+        try:
+            data = resp.json()
+        except Exception:
+            log.warning(f"  non-JSON response: {resp.text[:200]}")
+            return []
         log.debug(f"  json type: {type(data).__name__}, keys: {list(data.keys()) if isinstance(data, dict) else f'list[{len(data)}]'}")
         # Gelbooru wraps posts in {"post": [...]} or returns {"post": []}
         if isinstance(data, dict):

booru_viewer/core/api/moebooru.py

@@ -23,7 +23,10 @@ class MoebooruClient(BooruClient):
 
         resp = await self.client.get(f"{self.base_url}/post.json", params=params)
         resp.raise_for_status()
-        data = resp.json()
+        try:
+            data = resp.json()
+        except Exception:
+            return []
         if isinstance(data, dict):
             data = data.get("posts", data.get("post", []))
         if not isinstance(data, list):