feat: Add component tests for components/ui/select.tsx

[Jayydy]

Closes #651

The implementation ha been completed. Here's what the test file covers and why each choice was made:

Placeholder (3 tests)

• Asserts the placeholder text renders when no value is set
• Checks data-placeholder is present on the trigger — this is the attribute
  base-ui sets and what the CSS data-placeholder:text-muted-foreground rule
  targets, so testing it verifies the contract the component's styling depends
• Confirms both the attribute and the text are absent once defaultValue is
  provided

Value selection (3 tests)

• Uses userEvent.setup() + click to open the popup (portalled into
  document.body) then selects an item via within(listbox)
• Asserts onValueChange is called with the correct string value
• Asserts data-placeholder is removed and the trigger text updates after a
  controlled rerender — this mirrors how the component is used in forms
  throughout the app

Disabled state (4 tests)

• Checks the native disabled attribute (makes toBeDisabled() true)
• Checks data-disabled — the attribute that drives disabled:cursor-not-allowed
  disabled:opacity-50 in the trigger's className
• Verifies clicking a disabled trigger doesn't open the popup (no listbox role
  appears)
• Verifies onValueChange is never called when disabled

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