Only show message to stop instance when instance has been selected

[charliepark]

A small logic update to check for the presence of a selected instance before showing the "plz stop instance" message box. Also updates the copy from "An instance" to "This instance"

Closes #2943

No selected instance:

A not-stopped instance has been selected:

O ho! A stopped instance has been selected:

[vercel]

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Project	Deployment	Preview	Updated (UTC)
console	Ready	Preview	Nov 4, 2025 5:22am

[charliepark]

I think adding a test here is overkill, but on the off chance we want to add one, this one is ready to go.

test('add instance modal shows notice only for non-stopped instances', async ({ page }) => {
  await page.goto('/projects/mock-project/affinity/oil-water')

  const addInstanceButton = page.getByRole('button', { name: 'Add instance' })
  const addInstanceModal = page.getByRole('dialog', { name: 'Add instance to group' })
  const instanceCombobox = page.getByRole('combobox', { name: 'Instance' })
  const notice = page.getByText('An instance must be stopped to add it to a group')
  const modalAddButton = page.getByRole('button', { name: 'Add to group' })

  // Open modal - notice should not be visible when no instance is selected
  await addInstanceButton.click()
  await expect(addInstanceModal).toBeVisible()
  await expect(notice).toBeHidden()

  // Select a running instance - notice should appear and button should be disabled
  await page.getByRole('option', { name: 'db1' }).click()
  await expect(instanceCombobox).toHaveValue('db1')
  await expect(notice).toBeVisible()
  await expect(modalAddButton).toBeDisabled()

  // Close modal and stop db1
  await page.getByRole('button', { name: 'Cancel' }).click()
  await expect(addInstanceModal).toBeHidden()
  await page.getByRole('link', { name: 'Instances' }).click()
  await clickRowAction(page, 'db1', 'Stop')
  await page.getByRole('button', { name: 'Confirm' }).click()
  await expectRowVisible(page.getByRole('table'), {
    name: 'db1',
    state: expect.stringContaining('stopped'),
  })

  // Go back to the anti-affinity group
  await page.getByRole('link', { name: 'Affinity' }).click()
  await page.getByRole('link', { name: 'oil-water' }).click()

  // Open modal again and select the now-stopped instance
  await addInstanceButton.click()
  await expect(addInstanceModal).toBeVisible()
  await page.getByRole('option', { name: 'db1' }).click()
  await expect(instanceCombobox).toHaveValue('db1')

  // Notice should NOT appear and button should be enabled for stopped instance
  await expect(notice).toBeHidden()
  await expect(modalAddButton).toBeEnabled()

  // Close modal
  await page.getByRole('button', { name: 'Cancel' }).click()
  await expect(addInstanceModal).toBeHidden()
})