feat(ui): present page-action editors as sheets on mobile

The Add-tags and Add-to-collection editors opened as centered Modals on
every form factor, so a mobile user went action-Sheet → centered Modal — an
inconsistent hop, and AddTagsSheet's docstring promised a sheet it never
got. Introduce AdaptiveDialog, which renders a bottom Sheet under 640px and
a centered Modal above it behind one shared contract (open / title / onClose
/ children / testid). Route both editors through it:

- AddToCollectionModal now wraps AdaptiveDialog (both its call sites — manga
  detail and the reader — adapt automatically).
- The reader hosts AddTagsSheet in an AdaptiveDialog instead of a Modal, so
  its "slots into a Sheet (mobile) or Modal (desktop)" contract is now true.

Modal/Sheet already share an API, so the switch is transparent: the dialog
keeps the same role, title, testid, and close affordance. Desktop is
unchanged.

AdaptiveDialog has its own unit test (Sheet on mobile, Modal on desktop);
an e2e proves the mobile long-press → Add-tag flow now opens a Sheet.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

frontend/e2e/page-context-menu.spec.ts

@@ -373,5 +373,12 @@ test.describe('page action sheet (mobile long-press)', () => {
         await expect(page.getByTestId('page-action-add-tag')).toBeVisible();
         await expect(page.getByTestId('page-action-save-image')).toBeVisible();
         await expect(page.getByTestId('page-action-copy-link')).toBeVisible();
+
+        // The editor itself opens as a bottom Sheet on mobile (not a centered
+        // Modal), so the action-sheet → editor flow stays one consistent
+        // idiom. Sheet renders a `${testid}-scrim`; Modal a `${testid}-backdrop`.
+        await page.getByTestId('page-action-add-tag').click();
+        await expect(page.getByTestId('add-tags-modal-scrim')).toBeVisible();
+        await expect(page.getByTestId('add-tags-modal-backdrop')).toHaveCount(0);
     });
 });