Replace kmail with thunderbird

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: shanemcd

mybox/Containerfile

@@ -100,8 +100,7 @@ RUN <<EORUN
         \
         `# Desktop applications` \
         chromium \
-        kaccounts-integration \
-        kmail \
+        thunderbird \
         qt6-qttools \
         Sunshine \
         \