fix(images): explain why Set category has nothing to set

Set category on a selection where no image is linked to a Library prompt
said only "None of the selected images has a linked prompt." It now says
why and what it means: "The selected images aren't in your Library yet,
so there is no prompt category to set." Unit and page-workflow tests
follow the copy.

tests/browser/workspace-images.spec.mjs

@@ -1921,7 +1921,7 @@ test('Set category without a linked image posts nothing and opens no dialog', as
     await page.route(categoryRoute, route => { posted += 1; return route.continue(); });
     await checkbox(cards(page).last(), 'same.png').check();
     await categoryButton(page).click();
-    await expect(toast(page)).toHaveText('None of the selected images has a linked prompt.');
+    await expect(toast(page)).toHaveText("The selected images aren't in your Library yet, so there is no prompt category to set.");
     await expect(page.locator('dialog[open]')).toHaveCount(0);
     expect(posted).toBe(0);
     await expect(bulkbar(page).locator('.bulkbar > strong')).toHaveText('1 selected');

tests/js/workspace-images-category.test.js

@@ -33,7 +33,7 @@ test('the outcome toast counts the prompts and the skipped images', async () =>
 test('the dialog and failure copy is the plan\'s', async () => {
     const {CATEGORY_DIALOG, CATEGORY_NONE_LINKED, CATEGORY_FAILED} = await source;
     assert.deepEqual(CATEGORY_DIALOG, {title:'Categorize selected images', label:'Category', action:'Apply'});
-    assert.equal(CATEGORY_NONE_LINKED, 'None of the selected images has a linked prompt.');
+    assert.equal(CATEGORY_NONE_LINKED, "The selected images aren't in your Library yet, so there is no prompt category to set.");
     assert.equal(CATEGORY_FAILED, 'Category could not be set. Try again.');
     assert.ok(Object.isFrozen(CATEGORY_DIALOG));
 });

web/js/workspace/images-category.mjs

@@ -5,7 +5,7 @@ import { countOf, noun } from './plural.mjs';
  * filter matches the linked positive prompt's, so the action sets the category on those prompts.
  */
 export const CATEGORY_DIALOG = Object.freeze({title:'Categorize selected images', label:'Category', action:'Apply'});
-export const CATEGORY_NONE_LINKED = 'None of the selected images has a linked prompt.';
+export const CATEGORY_NONE_LINKED = "The selected images aren't in your Library yet, so there is no prompt category to set.";
 export const CATEGORY_FAILED = 'Category could not be set. Try again.';
 // The server's limit (`validate_category`): the field stops there instead of failing on Apply.
 export const CATEGORY_MAX_LENGTH = 100;