Leave unchanged JSON locale files alone and keep their trailing newline

[arvida]

Bug

Every JSON write re-serialised the whole file with JSON.stringify(x, null, 2) and no trailing newline, even when nothing changed. In a sync the source-language cells are part of the payload, so a hand-formatted en.json was rewritten on every sync: inline arrays and one-line objects got exploded, and git showed \ No newline at end of file (newsroom-demo PR #19). The delete path had the same problem on a larger scale: pull calls it for every translation file when keys were deleted, and each JSON file was rewritten even when it held none of those keys.

Change

All in src/utils/translation-updater/json-handler.ts:

• updateJsonFile skips the write when the merged content serialises exactly like what was read (compact JSON.stringify on both, so key order counts too). The skip only happens when a write would have changed nothing but whitespace. A file that is new or unparseable is always written.
• deleteKeysFromJsonFile returns without writing when none of the keys were in the file.
• When a file is written, it keeps its trailing newline if it had one. New files get one. A file without one keeps none.
• Return values are unchanged (updatedKeys, created, deleted keys), so counts and summaries stay the same.

Not changed: indentation is still 2 spaces, and a file that really does change still has inline arrays expanded. Keeping those would need a splicer like yaml-splicer.ts.

Plan with a caller audit (translate, ci sync in git and signed-commit mode, pull update and delete, source alignment, multi-language files, new files): dev-docs/specs/859-json-write-noise.md in the monorepo. Codex reviewed the plan and found no blockers.

Verified

• New tests in tests/utils/translation-updater/json-write-noise.test.ts were written first: 6 of the 7 failed on main's code. The 7th covers a file without a trailing newline, which main already left alone. They cover a byte-identical hand-formatted file with unchanged values, a byte-identical multi-language file, the newline kept after a real change, no newline added to a file without one, a new file ending in a newline, a delete leaving a file without those keys untouched, and a delete keeping the newline.
• npm test: 83 suites, 1502 tests pass. npx tsc --noEmit and npx eslint 'src/**/*.ts' are clean.
• Built dist and ran updateTranslationFile on a copy of newsroom-demo's apps/editor en.json with its own values, which is what a sync does. The file stayed byte-identical (34 lines in, 34 out). A real change to sv.json updated the value and kept the newline.
• Codex review of the diff: no findings. A separate code-reviewer pass: no blockers.

Known edge case, not a blocker: a file with a duplicated key is no longer cleaned up by a no-op write, since the write is skipped.

Fixes localheroai/localhero-ai#859