Align BankID app autostart with current BankID guidelines

[torselden]

Updates how Active Login launches the BankID app so it follows BankID's current autostart guidance. Autostart is the default on all supported devices, the recommended universal/app link is used on mobile, and the manual launch button is only shown as a fallback where a browser actually requires user interaction.

This PR fixes #534

High level overview of this PR

• Use the universal/app link (https://app.bankid.com/) for all iOS and Android browsers, and keep the bankid:/// scheme for desktop
• Launch the app via BankID's recommended anchor-click with referrerPolicy="origin" instead of window.location.href (fixes unreliable iOS universal-link launch)
• Restrict the manual "start BankID" fallback button to Android browsers that require a user gesture (Chrome, Edge, Samsung Internet, Brave); keep autostart automatic on iOS and on Android Firefox/Opera

These things have been implemented (when relevant)

• Code written
• Test added — launcher tests for app-link vs scheme selection, the user-interaction matrix, and redirect parameter behavior (Android redirect=null, iOS Safari return URL, iOS Chrome/Firefox browser scheme, iOS Edge/Opera redirect=null)
• Documentation updated / written — added a "How the BankID app is launched" section and a mobile browser-support note in bankid.md

Backwards-compatibility note
This PR changes the launch URL selection for several device/browser combinations (no public API changes):

• CanUseAppLink no longer restricts the app link to Safari-on-iOS and Chrome/Edge-on-Android ≥ 6 — it now applies to every iOS and Android browser. The DeviceOsVersion.MajorVersion >= 6 guard was removed.
  As a result, iOS non-Safari (Chrome, Firefox, Edge, Opera) and Android Firefox/Opera / older Android now receive https://app.bankid.com/… where they previously received bankid:///…. This matches current BankID guidance and should improve reliability.

• The manual "start BankID" fallback button is now scoped to restricted Android browsers instead of a broader set.
  What consumers should verify: if you relied on the old scheme-based launch for a specific browser, that path changes. Android Firefox/Opera in particular should be verified on real devices, since their historical app-link/intent handling has been inconsistent. Custom IBankIdLauncher / IBankIdSupportedDeviceDetector implementations are unaffected, and the deprecated redirect query parameter is still emitted, so BankID app compatibility is preserved.