chore: add config.json to .gitignore

config.json is managed directly on each deploy host (contains the GIF
API key and homeserver list) and is intentionally not committed. It was
already untracked so git reset --hard left it alone, but git clean would
have deleted it. Adding it to .gitignore makes the intent explicit and
protects it from accidental git clean runs.

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

.gitignore

@@ -5,3 +5,8 @@ devAssets
 
 .DS_Store
 .ideapackage-lock.json
+
+# Runtime config — managed directly on each deploy host, never committed.
+# Contains the GIF API key and homeserver list. git reset --hard won't touch
+# it but git clean would delete it without this entry.
+config.json