feat(example-images): add missing-only download path and skip existing files

Split the single-model and bulk context menu actions into 'Download
Missing Example Images' (regular endpoint, skips already-processed
models) and 'Re-process Example Images' (force endpoint, retries
failed models).

- start_download accepts model_hashes so a selected subset can be
  processed with the progress-aware skip logic; explicitly targeted
  models bypass the failed/processed model-level guards so per-image
  gaps are filled
- pre-download existence check in the processor skips network requests
  for image files already on disk across all download paths
- force download retries previously failed models and clears their
  failed status on success
- add i18n keys for the new menu items across all locales

locales/en.json

@@ -772,7 +772,8 @@
       "unfavorite": "Remove from Favorites",
       "deleteAll": "Delete Selected",
       "downloadMissingLoras": "Download Missing LoRAs",
-      "downloadExamples": "Download Example Images",
+      "downloadMissingExamples": "Download Missing Example Images",
+      "reprocessExamples": "Re-process Example Images",
       "clear": "Clear Selection",
       "skipMetadataRefreshCount": "Skip ({count} models)",
       "resumeMetadataRefreshCount": "Resume ({count} models)",
@@ -807,7 +808,8 @@
       "sendToWorkflowAppend": "Send to Workflow (Append)",
       "sendToWorkflowReplace": "Send to Workflow (Replace)",
       "openExamples": "Open Examples Folder",
-      "downloadExamples": "Download Example Images",
+      "downloadExamples": "Download Missing Example Images",
+      "reprocessExamples": "Re-process Example Images",
       "replacePreview": "Replace Preview",
       "setContentRating": "Set Content Rating",
       "moveToFolder": "Move to Folder",