Compares all locale bundles against en_US reference,
reports missing keys per locale and namespace.

Renamed copy:locales → i18n:copy, added i18n:check.
All i18n scripts now live under scripts/i18n/.

- Add common group for shared sidebar concepts (inbox, favorites, trash, etc.)
- Nest spaces keys (spaces.code.label instead of spaces.code)
- Move mathNotebook keys into spaces.math
- Remove ~25 unused keys (button.clear, snippet.plural, loading, etc.)
- Delete sidebar and mathNotebook groups (all migrated)

Update all i18n.t() calls to match new key structure:
- sidebar.* → common.* / spaces.{id}.*
- spaces.code → spaces.code.label (flat → nested)
- mathNotebook.* → spaces.math.*
- fragment → common.fragment
- total → common.total

- Apply same ui.json restructure to 16 locales (common, nested spaces)
- Add missing keys in menu.json, messages.json, preferences.json
  with en_US fallback values
- All locales now fully in sync with en_US reference

Translate all English fallback values in ru_RU locale:
- ui.json: spaces labels, notes, actions, placeholders
- menu.json: mathNotebook, window
- messages.json: language reload description
- preferences.json: notesEditor section, theme options

Add section describing the spaces-based key hierarchy
and a quick reference table for where to place new keys.