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/lib/components/AddToCollectionModal.svelte

@@ -1,5 +1,5 @@
 <script lang="ts">
-    import Modal from './Modal.svelte';
+    import AdaptiveDialog from './AdaptiveDialog.svelte';
     import {
         addMangaToCollection,
         createCollection,
@@ -166,7 +166,7 @@
     }
 </script>
 
-<Modal {open} {onClose} title="Add to collection" size="md" testid="add-to-collection-modal">
+<AdaptiveDialog {open} {onClose} title="Add to collection" size="md" testid="add-to-collection-modal">
     {#if loading}
         <p class="status">Loading your collections…</p>
     {:else if error}
@@ -235,7 +235,7 @@
             <span>{creating ? 'Creating…' : 'Create + add'}</span>
         </button>
     </form>
-</Modal>
+</AdaptiveDialog>
 
 <style>
     .status {