Fix example loading from URL query parameter

[Copilot]

When navigating to the playground with ?example=math, syntax highlighting wasn't applied and results showed stale values until manual edit.

Root Cause

In app.js, loadExampleFromUrl() was called before onDidChangeContent handlers were registered. When setValue() was called on the models, no handlers existed to trigger highlighting or evaluation.

Changes

• Reordered initialization in app.js: register event handlers before calling loadExampleFromUrl()

// Before: handlers registered after example load (broken)
applyHighlighting();
evaluate();
loadExampleFromUrl();  // setValue() triggers nothing
expressionModel.onDidChangeContent(...);  // too late

// After: handlers registered first (fixed)
expressionModel.onDidChangeContent(...);
applyHighlighting();
evaluate();
loadExampleFromUrl();  // setValue() now triggers handlers

Original prompt

When navigation to an example with the "example" query parameter, ensure the example is evaluated and the expression is syntax-hightlighted. Right now, no syntax hightlighting in the monaco editor is visible and the result always shows "100" until an edit is made in the expression monaco editor

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[Copilot]

Pull request overview

This PR fixes the playground behavior when loading an example via the ?example= URL query parameter so that syntax highlighting and evaluation occur immediately without requiring a manual edit.

Changes:

• Reordered initialization to register onDidChangeContent handlers on expressionModel and contextModel before performing any initial highlighting/evaluation.
• Moved the loadExampleFromUrl() call to occur after event handlers are set up so that setValue() correctly triggers highlighting and evaluation.
• Kept an explicit initial applyHighlighting() and evaluate() call to handle the default content load path.

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