Equation editor - #1515

mathlive's package.json is not exposed by its exports map, so
require('mathlive/package.json') throws ERR_PACKAGE_PATH_NOT_EXPORTED
and breaks the webpack config load (caught by pre-build review).

Live feedback round: raw i18n keys, import failure, blocking modal,
stuck virtual keyboard, lost drag/minimize, trailing spaces on export.

- Root cause of raw keys: webpack translations-loader filters locale
  JSON down to keys listed in frontend/extracted-translations.json;
  regenerate with yarn extract-translations (all 27 module keys now
  extracted) and add the 8 v5 strings to locales/en.json
- Restore the old commit's non-blocking floating window (draggable,
  minimize/restore, no backdrop/focus-trap) so the MathLive virtual
  keyboard on document.body stays usable; z-indexes from the old
  stylesheet (.ML__keyboard above .latex-editor-modal-root)
- Import now handles both a non-empty selection and an
  equation-under-cursor (mathAncestorNode/parseMathContainer, same
  extraction the math preview tooltip uses); CM access stays on
  CodeMirrorViewContext
- wrapLatex trims whitespace and leading/trailing MathLive
  text-space tokens (\text{ }); unit tests added
- mathlive-input back on the proven init flow using debugConsole

MathLive CSS, nested font copy

- 'Open in Equation Editor': the toolbar button listens on window
  (useEventListener hook) but the tooltip dispatched a non-bubbling
  document event that never arrived; dispatch now bubbles
- mathlive/static.css (math-field + KaTeX font-faces) was missing from
  the v5 stylesheet rewrite; restore the @import
- CopyPlugin nested the KaTeX fonts into fonts/fonts/ while MFE.fontsDirectory
  pointed at the outer dir (404 -> CDN fallback); copy to
  js/libs/mathlive-<version> so files land at <version>/fonts/

Implements the round-3 acceptance criterion: 'import an equation (or
open it via Open in Equation Editor) and press Export directly
afterwards - the equation with its environment is intact and the same'.

- splitEquation(): pure util that splits an equation as it appears in
  the document into body + wrapper ($...$ -> inline, $$/\[ \] ->
  display, \begin{equation*?} -> equation, \begin{eqnarray*?} ->
  eqnarray); no fence -> new default wrapper 'inline'
- modal default export wrapper is now 'inline'; pre-loaded content and
  imports are split into body (math-field) + detected environment
  (select), so wrapLatex(body, wrapper) reproduces the original
- 'Open in Equation Editor' dispatches the equation's document range;
  the button selects that range once open (visible mark), so Export
  replaces the original equation exactly; clicking elsewhere moves the
  selection and Export then inserts at the new cursor (no stale ranges)
- tooltip canonicalises display math to \[...\] (the display wrapper
  form) before dispatch
- 10 new unit tests incl. round-trip canonical-form checks