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/src/routes/manga/[id]/chapter/[chapter_id]/+page.svelte

@@ -11,7 +11,7 @@
     import { readerFullscreen } from '$lib/reader-fullscreen.svelte';
     import { session } from '$lib/session.svelte';
     import Sheet from '$lib/components/Sheet.svelte';
-    import Modal from '$lib/components/Modal.svelte';
+    import AdaptiveDialog from '$lib/components/AdaptiveDialog.svelte';
     import SegmentedControl from '$lib/components/SegmentedControl.svelte';
     import TapZone from '$lib/components/TapZone.svelte';
     import PageContextMenu from '$lib/components/PageContextMenu.svelte';
@@ -1555,7 +1555,7 @@
                 if (activePageId) void loadPageSummary(activePageId);
             }}
         />
-        <Modal
+        <AdaptiveDialog
             open={tagsModalOpen}
             title="Tag this page"
             onClose={() => (tagsModalOpen = false)}
@@ -1568,7 +1568,7 @@
                     activePageTags = tags;
                 }}
             />
-        </Modal>
+        </AdaptiveDialog>
     {/if}
 {/if}