DBA: replace CodeMirror 5 with CodeMirror 6 + Lezer XQuery

[GuntherRademacher]

The changes here replace the DBA's CodeMirror 5 editor with CodeMirror 6, driven by the Lezer-based XQuery language pack:

• Lezer syntax highlighting, code folding, autocomplete, and function tooltips with documentation links
• parse-error gutter (error markers + underlines) from the Lezer tree
• Material-Light / One-Dark themes (as in the Fiddle), following prefers-color-scheme
• XML highlighting for the XML panels (query output, resource editor, Jobs viewer, Users)

Changes in basex-api (dba/):

• add static/codemirror6/cm6.js - a self-contained IIFE (window.CM6) bundling CM6 + the XQuery language pack + @codemirror/lang-xml + themes + the parse-error gutter. Replaces the CM5 files.

• static/js.js - loadCodeMirror rewritten to build CM6 behind a CM5-compatible handle (setValue/getValue/historySize/clearHistory/setCursor/setOption/focus + a hidden-textarea mirror), so editor.js is untouched. Non-resized editors (Users/User) get the fixed 300px height CM5 used; Android/no-CM6 textarea fallback kept.

• lib/html.xqm - swap the three CM5 <script> + CSS <link> for the single cm6.js; add img-src 'self' data: to the CSP (lint markers are inline data: SVGs)

• static/style.css - CM5 .CodeMirror* rules replaced by CM6.

• remove static/codemirror/* (CM5 assets).

• cm6.js - the bundle is built outside this repo in nuxt-codemirror-lezer/dba-bundle as one IIFE, which is committed here.

Licensing: all bundled libraries are MIT; the minified cm6.js carries a /*! … */ banner with their copyright + MIT notices (as the old codemirror.js did).

[ChristianGruen]

(merged)