feat(glossary): add episode groups, search, and import/export

Apply glossary edits directly to translated subtitles and reload the active track. Keep terms with their originating episode, preserve existing translations, and clear stale episode groups with glossary entries.

README.md

@@ -90,6 +90,24 @@ AppImage only runs on distros whose glibc is at least as new as the build machin
 Translations are cached in `~/.cache/qtpotato/translations/` as `<video>.<key>.<lang>.srt`.
 Only text subtitles can be translated; image-based ones (PGS, VobSub) are not supported.
 
+Use **Subtitles → Edit Translation Glossary…** to back up or transfer terms:
+
+- **Export…** saves the full current list and episode groups to a JSON file, including edits
+  you have made in the dialog. The search filter does not limit the export.
+- **Import…** merges a JSON glossary into the dialog. Existing Original terms take precedence
+  (duplicates are compared without case differences), and episode groups are preserved.
+  Review the result and click **Save** to apply it, or **Cancel** to discard it.
+
+Importing and exporting glossary files do not call the translation API.
+
+Terms extracted while translating an episode are stored under that episode. Later episodes
+reuse all existing terms and keep their original ownership; only newly extracted terms are
+added to the current episode. Old or imported terms without provenance are assigned to the
+first episode in which the glossary is opened, because their original episode cannot be
+reconstructed. Your edited translations and existing episode assignments are preserved.
+Groups sort by episode number, and changing group assignments does not invalidate the
+translation cache.
+
 ## Keyboard shortcuts
 
 The defaults follow Haruna.