feat(ui): add onboarding and token-based themes

Introduce StartRouter to gate first run with onboarding. Persist state in
SharedPreferences (onboarded_v1). Add WelcomeScreen, a 3-step
OnboardingFlow, and a SignInScreen placeholder with Apple/Google buttons
and "coming soon" toasts plus continue as guest.

Apply new AppThemes (light/dark, high-contrast option) built from design
tokens and wire into MaterialApp (theme/darkTheme, system mode). Add
ThemeProvider to persist ThemeMode and high-contrast flags (not yet
hooked into the tree).

Add EN/MS i18n strings for welcome, onboarding, auth, and CTAs. Include
assets/tokens/design-tokens.json for design tooling parity.

assets/lang/en.json

@@ -78,5 +78,23 @@
   "settings.theme.light": "Light",
   "settings.theme.dark": "Dark",
   "lang.en": "English",
-  "lang.ms": "Bahasa Malaysia"
+  "lang.ms": "Bahasa Malaysia",
+  "welcome.title": "Spot it. Snap it. Fix it.",
+  "welcome.subtitle": "Report city issues in seconds—help crews act faster.",
+  "cta.continueGuest": "Continue as guest",
+  "cta.signIn": "Sign in",
+  "cta.skip": "Skip",
+  "cta.next": "Next",
+  "cta.getStarted": "Get started",
+  "onboarding.header": "Welcome to FixMate",
+  "onboarding.title1": "Fast capture",
+  "onboarding.body1": "Take a photo and submit in under a minute.",
+  "onboarding.title2": "Map clarity",
+  "onboarding.body2": "See issues around you with smart clustering.",
+  "onboarding.title3": "Track progress",
+  "onboarding.body3": "Follow status updates as work gets done.",
+  "auth.title": "Sign in",
+  "auth.signInWithApple": "Sign in with Apple",
+  "auth.signInWithGoogle": "Sign in with Google",
+  "auth.comingSoon": "Coming soon"
 }