Enhance code editor typing experience (bracket matching, auto-close, resize)

[enkr1]

Problem

The code editor in programming assessments lacks basic IDE typing features and has a fixed, non-resizable height.

Changes

IDE typing features (EditorField.tsx)

• Auto-close brackets/quotes (behavioursEnabled)
• Wrap selection with brackets/quotes (wrapBehavioursEnabled)
• Multi-cursor via Cmd+D / Ctrl+D (enableMultiselect)
• Active line + selected word highlighting
• Remove print margin line

Autocomplete explicitly OFF — exam fairness preserved.

Resizable editor (Editor.jsx)

• Replace fixed 25-line height with CSS resize: vertical drag handle
• ResizeObserver keeps Ace in sync with container size
• Default 400px, min 200px

Tests

• Verify IDE options enabled
• Verify autocomplete options are false

Not in scope

• Autocomplete (exam integrity)
• Editor library swap (future — CodeMirror 6 or Monaco)

Related: #6400, #307