Bug fix — keyboard focus lost after custom actions (txt2tags-it.qml)

[luginf]

After invoking a custom action via keyboard shortcut (e.g. "Heading 2"), the editor lost focus, forcing the user to
click with the mouse before typing again.

• Root cause: the toolbar/menu button steals focus when the action fires.
• First attempt: script.noteTextEditSetFocus() — does not exist, throws TypeError on v26.5.14.
• Fix: mainWindow.focusNoteTextEdit() added at the end of customActionInvoked(). This is the only official API method for restoring editor focus.

noteToMarkdownHtmlHook optimisations (hot path, called on every keystroke):

• _isWindows — platformIsWindows() cached in init() (avoids an external call on every render)
• _headRe — regex /[\s\S]?</head>/ pre-compiled in init() (no more new RegExp(...) on each call + (?:.|\n)? replaced by [\s\S]*? which is significantly faster on large HTML)
• _urlAttrRe — URL attribute regex pre-compiled in init() + lastIndex = 0 reset before each replace()
• _cssInject — constant CSS string extracted as a property

txt2tags_autolink optimisation (inline rule, called at every position):

• charCode fast-fail before src.slice(pos) + exec() — avoids creating a substring for every / and any non-letter character