chore(tooling): generate_localization.dart leaves the ARB files without a trailing newline

[marassteiner]

Running the documented setup from CONTRIBUTING.md on a clean checkout leaves the working tree dirty: dart run tool/generate_localization.dart rewrites assets/languages/strings_de.arb and assets/languages/strings_en.arb without their trailing newline.

Reproduction

git checkout staging && git status --short   # clean
flutter pub get
dart run tool/generate_localization.dart
git status --short
#  M assets/languages/strings_de.arb
#  M assets/languages/strings_en.arb

The diff is the same in both files:

   "youSell": "You sell"
-}
+}
\ No newline at end of file

Reproduced twice on a clean tree, so it is the generator writing the ARB files back, not an editor or a pre-commit hook.

Why it is worth fixing

The generator's job is lib/generated/i18n.dart (gitignored, CONTRIBUTING.md line 162) — rewriting the committed source ARB files is a side effect. Every contributor who follows the documented setup gets two modified files they did not touch, and the next git add -A silently commits the newline removal back and forth.

Suggested fix

Have the generator either leave the ARB files untouched, or write them back with a trailing newline so a re-run is a no-op.

Context

Found while reviewing #897 (the PR itself does not touch this — it only changes one golden test and its baseline). Pre-existing, not a merge blocker there.