feat(filters): show a tag's own colour on its active filter chip (#185)

A tag carries a colour, and every place a tag appears shows it — a product card, the filter drawer's control, the admin taxonomy screen — except the removable chips beside the Filters button, which rendered every filter as a default grey. Picking `vintage` from a control that showed it in red produced a grey chip of the same name right next to it.

Only tags get a colour, because only tags have one. Category, price, favorites and status keep the default, and that asymmetry is the point: in a row mixing four kinds of filter, colour now means "this is a tag". Nothing depends on it — every chip still carries its label — so this reads the same to anyone who cannot distinguish the colours.

The close control inherits the tag's text colour, so a coloured chip gets a matching cross rather than a grey one on a coloured ground. A tag not yet in the loaded options has no colour to use and keeps the default, which is the same window the existing `Tag {id}` label fallback covers.

The test asserts the chip's colour equals the same tag's colour on a product card, rather than asserting it is red. The colour is derived from the tag's name and free to change; what must hold is that a tag looks like itself wherever it appears, and comparing the two places says that directly. Confirmed to fail without the change — the old grey chip sets no colour class at all.

Verified visually as well as by assertion: three tags selected together render in the drawer, in the chip row and on the card in the same colours.

Closes #185

frontend/tests/e2e/pages/StorefrontPage.ts

@@ -135,6 +135,22 @@ export class StorefrontPage {
     return this.page.getByRole('button', { name: `Remove filter ${name}` });
   }
 
+  /**
+   * The chip itself rather than its remove button, for asserting how it looks.
+   *
+   * antd expresses a tag's colour as an `ant-tag-<colour>` class, so the chip
+   * element is what carries it — the button inside only inherits the text
+   * colour.
+   */
+  filterChip(name: string): Locator {
+    return this.activeFilters.locator('.ant-tag').filter({ hasText: name });
+  }
+
+  /** A tag as rendered on a product card, which is where its colour is set. */
+  cardTag(itemName: string, tagName: string): Locator {
+    return this.card(itemName).locator('.ant-tag').filter({ hasText: tagName });
+  }
+
   async clearAllFilters(): Promise<void> {
     await this.activeFilters.getByRole('button', { name: 'Clear all' }).click();
   }