Accessibility: Add screen reader support to the editor via QAccessibleTextInterface

[Copilot]

Blind and low-vision users could not use TeXstudio's editor with screen readers (NVDA, JAWS, Narrator) because the custom QEditor widget—based on QAbstractScrollArea with custom painting—exposed no accessibility tree, so AT tools had no way to read text content, track the caret, or detect changes.

New: QEditorAccessible (qeditoraccessible.h/.cpp)

Implements QAccessibleWidget + QAccessibleTextInterface for QEditor, wiring the editor into Qt's platform accessibility bridge (IAccessible2 on Windows):

• Role/state: EditableText, reflects readOnly and selection state
• Text content: QAccessible::Value returns the full document text; text(start, end) supports range queries
• Cursor & selection: cursorPosition(), setCursorPosition(), selection(), setSelection() — all convert between (line, column) and absolute offsets using \n-separated line counting
• Navigation support: textAtOffset/Before/After() with Char/Word/Line/Sentence/Paragraph boundary types via QTextBoundaryFinder
• Hit testing: characterRect(offset) maps document coordinates to screen-space; offsetAtPoint() does the reverse
• Factory: QEditorAccessible::factory() registered with QAccessible::installFactory() — Qt's bridge calls this to instantiate the interface on demand

Modified: qeditor.cpp/.h

• documentOffsetFromPosition(line, col) — converts (line, column) to absolute offset (used by both the accessible interface and event firing)
• emitCursorPositionChanged() fires QAccessibleTextCursorEvent with the current absolute offset when the caret moves
• updateContent() fires QAccessible::ValueChanged when document content changes
• Factory registered in QEditor::init(); events guarded by QAccessible::isActive() to avoid overhead when no AT is running

// Cursor movement notification example
if (QAccessible::isActive()) {
    const int cursorOffset = documentOffsetFromPosition(m_cursor.lineNumber(), m_cursor.columnNumber());
    QAccessible::updateAccessibility(new QAccessibleTextCursorEvent(this, cursorOffset));
}

[sunderme]

@copilot fix compilation error

[Copilot]

@copilot fix compilation error

Fixed in the latest commit. The methods were named textBeforeCursor / textAfterCursor but Qt5's QAccessibleTextInterface uses textBeforeOffset / textAfterOffset. Renamed both declarations in the header and definitions in the implementation file.