Fix React state mutation bug causing textbox issues in search panel

[Copilot]

Problem

When users edited textboxes in the search panel, React would lose cursor position or experience other controlled input issues. This was caused by improper state mutation in the search dialog component.

Root Cause

The updateExpression, addExpression, and deleteExpression methods in packages/sanddance-explorer/src/dialogs/search.tsx were violating React's immutability principle:

1. Only the top-level groups array was being shallow-copied
2. Nested group and expressions objects were being directly mutated from state
3. The updateExpression method used unnecessary deep cloning with SandDance.VegaMorphCharts.util.clone()

Example of the problematic pattern:

// Before - Mutating state directly
const groups = [...this.state.groups];
const group = groups[groupIndex];  // Reference to old state
group.expressions[index] = newExpression;  // Direct mutation ❌
this.setState({ groups });

This caused React's reconciliation to behave unpredictably with controlled inputs, resulting in cursor position loss and focus issues.

Solution

All three methods now properly implement immutable state updates at every nesting level:

// After - Proper immutability
const groups = [...this.state.groups];
const group = { ...groups[groupIndex] };  // New group object
const expressions = [...group.expressions];  // New expressions array
expressions[index] = { ...currentEx, ...partialEx };  // New expression object
group.expressions = expressions;
groups[groupIndex] = group;
this.setState({ groups });  // ✅

Changes

• updateExpression: Replaced deep clone with spread operators and added proper immutability for all nested objects
• addExpression: Added proper group and expressions array copying before modifications
• deleteExpression: Added proper group object copying before modifications

Benefits

• ✅ Fixes cursor position loss in search panel textboxes
• ✅ Proper React immutability patterns throughout
• ✅ Better performance (avoiding unnecessary deep clones)
• ✅ More maintainable and predictable code
• ✅ All existing functionality preserved

Testing

• All builds pass
• All linting passes
• All existing tests pass
• No breaking changes to functionality

Original prompt

There a react bug when user edits textbox in the search panel

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