Closes #31.

A four-page swipeable flow shown once on first launch, before the tabs
mount: welcome, gender, preferences, and a closing page. Answers are
written to the store as they are tapped, so going back and forth keeps
them, and a dev-menu row replays the flow without touching any data.

- Gender is required; Next stays disabled until it is answered. The two
  options are pastel cards with the stick-figure icons, labelled Brother
  and Sister.
- Preferences is an extensible opt-in checklist, currently just the
  Hanafi Asr sunnah, each row with an info sheet explaining what it
  changes. Stored as an id list, so a future preference needs no new
  persisted field and existing users default to it being unset. The same
  list is reachable afterwards under Settings > Preferences.
- Skip finishes the flow early and is offered on the pages either side of
  the gender question. It stays mounted but invisible elsewhere so the
  footer height never changes mid-swipe.
- One mascot is drawn above the pager rather than per page, and
  interpolates its position and size between the room each page reserves
  for it, so it glides with the swipe instead of jumping between pages.

The pager is hand-rolled on a Pan gesture rather than a paging
ScrollView, whose contentOffset origin under RTL differs between iOS and
Android; layout direction reduces to a single multiplier here. Pages are
centred columns with no row layouts, so they read the same either way -
which also covers a first launch in Arabic, where forceRTL has been set
but does not apply until the next launch.

Also fixes a fresh install defaulting to English rather than the device
language, which the store needed a narrowed language type for anyway.

@@ -20,0 +24,4 @@
+    "stepLabel": "الخطوة {{current}} من {{total}}",
+    "welcomeTitle": "أهلًا بك في مُحْسِن",
+    "welcomeBody": "أجب عن هذين السؤالين وابدأ استخدام التطبيق",
+    "genderTitle": "الجنس، من فضلك؟",