chore: ignore editor swap files

A stray .CLAUDE.md.swp got picked up by `git add -A` while writing the
contributing docs. The CLAUDE.md ignore is anchored (/CLAUDE.md), so it
never covered vim's sibling swap file.

.gitignore

@@ -50,6 +50,11 @@ google-services.json
 .DS_Store
 Thumbs.db
 
+# Editor swap/backup files
+*.swp
+*.swo
+*~
+
 # F-Droid local artifacts (the pipeline generates them in CI)
 /fdroid/