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>

backend/Cargo.lock

@@ -1517,7 +1517,7 @@ checksum = "c41e0c4fef86961ac6d6f8a82609f55f31b05e4fce149ac5710e439df7619ba4"
 
 [[package]]
 name = "mangalord"
-version = "0.88.0"
+version = "0.89.0"
 dependencies = [
  "anyhow",
  "argon2",

backend/Cargo.toml

@@ -1,6 +1,6 @@
 [package]
 name = "mangalord"
-version = "0.88.0"
+version = "0.89.0"
 edition = "2021"
 default-run = "mangalord"