Lower deployment target to iOS 17

[tashda]

Summary

Make Shellbee available on iOS 17 and up. Currently the app requires iOS 26, which excludes most of the installed base.

Approach

Drop the deployment target to iOS 17 and gate iOS 18 / iOS 26 APIs behind if #available so newer-OS users still get the native look (Liquid Glass, minimized search toolbar, mesh gradients, indefinite symbol effects, the new Tab { } builder), while iOS 17 users get sensible old-school equivalents.

Implementation

• New Shellbee/Shared/Compat/iOS26Compat.swift with one-line helpers:
  • glassEffectIfAvailable(in:) → .glassEffect (26) / .ultraThinMaterial background (17)
  • glassButtonStyleIfAvailable() / glassProminentButtonStyleIfAvailable() → .glass / .glassProminent (26) / .bordered / .borderedProminent (17)
  • minimizeSearchToolbarIfAvailable() → .searchToolbarBehavior(.minimize) (26) / no-op (17)
  • bounceSymbolEffectIfAvailable() → indefinite .symbolEffect(.bounce) (18) / no-op (17, since BounceSymbolEffect only conforms to IndefiniteSymbolEffect on 18+)
• Call sites migrated to helpers: LightControlCard, FastTrackBanner, InAppNotificationBanner, GroupListView, LogsView, DeviceListView, DocBrowserView, OnboardingView, OnboardingTestPage.
• HomeBackgroundGradient: MeshGradient (iOS 18+) with a LinearGradient fallback (file-local).
• MainTabView: split into if #available(iOS 18, *) for the new Tab { } builder vs. the classic .tabItem API on iOS 17.
• Widgets (InterviewActivityWidget, ConnectionActivityWidget): replaced .repeat(.continuous) (iOS 18+) with .repeating (iOS 17+).
• Shared/Compat/iOS26Compat.swift added to ShellbeeWidgetsExtension membershipExceptions (widget target doesn't need it and chokes on the bounce indefinite helper).

Test plan

• Build & smoke test on iOS 17 simulator (iPhone 15 / earliest available)
• Build & smoke test on iOS 18 simulator
• Build & smoke test on iOS 26 simulator — confirm Liquid Glass / minimized search toolbar / mesh gradient / indefinite bounce all still render natively
• Verify the floating tab bar on iOS 17 (classic .tabItem) doesn't visually break the in-app notification overlay positioning
• Run full test suite on iOS 17 and iOS 26