fix(thunderbird-nightly): add lint exceptions for screenshots and developer-name

flatpak-builder-lint now flags metainfo-missing-screenshots and
appstream-missing-developer-name as errors. These are expected for a
nightly/unofficial Flatpak — add them to exceptions.json alongside the
existing appid-filename-mismatch exception.

Assisted-by: Claude Sonnet 4.6 via OpenCode

Author: castrojo

flatpaks/thunderbird-nightly/exceptions.json

@@ -1,3 +1,7 @@
 {
-    "org.mozilla.thunderbird.nightly": ["appid-filename-mismatch"]
+    "org.mozilla.thunderbird.nightly": [
+        "appid-filename-mismatch",
+        "metainfo-missing-screenshots",
+        "appstream-missing-developer-name"
+    ]
 }