i18n(scanner): translate the reconcile walk progress strings

The walk-progress feature left exactly one [TODO: Translate] key behind,
common.scanProgress.walkFiles ("{count} files"), so this is the whole
pending set for all 9 locales. Each rendering reuses the locale's
stages.count_models noun for "files":

  zh-CN {count} 个文件          zh-TW {count} 個檔案
  ja    {count} 件のファイル      ko    파일 {count}개
  fr    {count} fichiers        de    {count} Dateien
  es    {count} archivos        ru    {count} файл(ов)
  he    {count} קבצים

{count} arrives pre-formatted (toLocaleString), so no locale adds digit
grouping, and the string stays a bare fragment: the root labels, the
parentheses and the " | " before the ETA are composed by
static/js/api/baseModelApi.js. ru uses файл(ов) because the counter ticks
live (notEmptyMessageCount precedent).

Also document the new surface in docs/i18n-translation-guidelines.md:
a status note plus the "Scan progress (walk phase)" terminology section
recording the per-locale renderings and the fragment/composition rule.

Verified: python scripts/sync_translation_keys.py --dry-run reports no
pending changes, pytest tests/i18n 20 passed, and
grep -c "TODO: Translate" locales/*.json is 0 everywhere.

locales/de.json

@@ -74,7 +74,7 @@
                 "minutes": "~{minutes} Min. verbleibend",
                 "hours": "~{hours} Std. {minutes} Min. verbleibend"
             },
-            "walkFiles": "[TODO: Translate] {count} files"
+            "walkFiles": "{count} Dateien"
         }
     },
     "onboarding": {