fix(export): let the keepsake download through X-Frame-Options on iOS

The keepsake download navigates a hidden, same-origin iframe (deliberately:
a top-level navigation to a 404/429 would unload the PWA). Caddy stamped a
site-wide `X-Frame-Options: DENY` that also covered the proxied `/api/*`.

Blink hands a `Content-Disposition: attachment` response to the download
manager at the network layer, so Chromium never noticed. WebKit enforces XFO
on the frame navigation first and aborts the load — so on iOS Safari, the
app's primary platform, tapping Download did nothing at all, silently.

Carve the two export endpoints out to SAMEORIGIN, which still blocks
cross-origin framing. Implemented as two disjoint matchers rather than an
override: Caddy applies the FIRST `header` directive outermost, so it wins on
write and a later, more specific `header` is silently ignored (verified
against the running test stack).

Also close the test gap that let this ship:

- `06-export` ran on chromium-desktop only; add it to `webkit-iphone`, the
  only engine that enforces XFO on the download frame.
- No test in the suite ever clicked a download button — every archive
  assertion used Node `fetch`, which has no frame and no XFO enforcement.
  Add a spec that clicks it and awaits a real `download` event. Verified
  falsifiable: with the blanket DENY reinstated it fails and reports the
  WebKit refusal as the cause.
- Fix `ExportPage`'s card-scoped locators, which matched nothing: the cards
  carry `class="card p-5"` (a Tailwind `@apply` component class), never the
  `rounded-xl` the page object looked for. This had left the "shows enabled
  download buttons" test red on main.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

e2e/page-objects/export-page.ts

@@ -37,12 +37,19 @@ export class ExportPage {
   /**
    * The "Download" button inside the card whose heading is `heading`.
    *
-   * Scoped to the card element (`div.rounded-xl`) rather than "any div containing the
-   * heading" — the latter also matches the page wrapper, which contains BOTH cards' buttons.
+   * Scoped to the card element rather than "any div containing the heading" — the latter
+   * also matches the page wrapper, which contains BOTH cards' buttons.
+   *
+   * The scope class is `div.card` (see the ZIP/HTML cards in
+   * frontend/src/routes/export/+page.svelte). It was previously `div.rounded-xl`, which
+   * matched NOTHING: `.card` is a Tailwind `@apply` component class
+   * (frontend/src/lib/styles/components.css) so the DOM only ever carries `class="card p-5"`
+   * — and the utility it applies is `rounded-2xl` anyway. Both card-scoped locators were
+   * therefore dead, which is why the "shows enabled download buttons" test was red.
    */
   private cardButton(heading: string): Locator {
     return this.page
-      .locator('div.rounded-xl')
+      .locator('div.card')
       .filter({ has: this.page.getByRole('heading', { name: heading, exact: true }) })
       .getByRole('button', { name: 'Download', exact: true });
   }