feat(onboarding): add /start mode chooser, fix pilot-profile-setup UX

- Fix missing icon for the free-tier/self-host pricing option
- Make selected cards visually obvious (checkmark badge, stronger border)
- Add Back navigation between onboarding steps
- Network-experience screen: button reads "No, not yet" instead of an
  ambiguous "Continue" when nothing is selected
- classroom-introduction reverts to Classroom-only content; the
  Classroom/Live ATC chooser moves to a new /start page instead
- Remove the "Skip to Classroom" button in favor of a subtle text link
- New /start page: choose Classroom or Live ATC after login (when
  there's no specific redirect target) or right after finishing
  onboarding. Shows a "start with Classroom" hint only for first-time
  arrivals from pilot-profile-setup. Live ATC is locked for 24h after
  registration.

shared/onboarding/config.ts

@@ -99,7 +99,7 @@ export const PRICING_PREFERENCE_OPTIONS: OnboardingOption<PricingPreference>[] =
   { value: 'one_time', label: 'One-time unlock — I own it', icon: 'mdi-lock-open-variant-outline' },
   { value: 'monthly', label: 'Small monthly add-on, always latest', icon: 'mdi-calendar-sync-outline' },
   { value: 'season_pass', label: 'Bundle / season pass', icon: 'mdi-ticket-confirmation-outline' },
-  { value: 'free_self_host', label: "I'd rather stay free-tier / self-host", icon: 'mdi-server-network-outline' },
+  { value: 'free_self_host', label: "I'd rather stay free-tier / self-host", icon: 'mdi-source-branch' },
 ]
 
 export const ONBOARDING_TOTAL_STEPS = 7