feat: Implement APKAM Onboarding Flow

[Zambrella]

What I did:

• Introduced APKAM onboarding to support per-app, per-device PKAM keypairs.
• Added OnboardingApkamDialog widget to handle APKAM-specific enrollment flow.
• Modified the onboarding button logic to allow users to choose between traditional atKeys file upload and APKAM enrollment.
• Added loading to state to starting page button
• Added localization support with relevant strings for APKAM onboarding.

How I did it:

• Created a new widget onboarding_apkam_dialog.dart to manage APKAM enrollment, handling OTP submission, approval, and denial statuses.
• Updated onboarding_button.dart to prompt users to choose between APKAM or atKeys onboarding and handle their respective flows.
• Integrated device_info_plus to fetch device name.

- How to verify it

1. Launch the onboarding flow.
2. Upon selecting an activated atSign, the app should prompt you to choose between uploading an atKeys file or enrolling via APKAM.
3. Selecting APKAM:

• Enter the OTP from the app managing your keys (or use at_activate interactive <atsign> -> otp)
• Approve or deny (approve -i <enroll-id>)
• Ensure the app transitions through pendingApproval, success, or denied states as expected.

6. Verify the enrollment approval or denial from another app that holds manager keys.
7. Check that the app stores the appropriate keys post-enrollment and can authenticate successfully.
8. Confirm fallback to the traditional atKeys upload works without issues.

N.b. Would also be good to test the above but where the onboarding flow is interrupted part way through.

- Description for the changelog
Added APKAM onboarding flow

[Zambrella]

Although still in draft, if someone (@XavierChanth, @CurtlyCritchlow) could have a quick look and test of the logic that would be great.