fix: improve mobile alert details and notification navigation

[CoooderSan]

Summary

Improve the mobile alert-details experience and notification deep-link flow, rebased onto release/2.3 and aligned with the final AlertGroup API contract.

Changes

• Make alert details responsive on narrow screens, including long titles, labels, messages, and detail values.
• Display alert details as a full-screen, theme-consistent view on mobile devices.
• Lock the page behind an open modal and prevent scroll chaining at modal boundaries.
• Open notification links directly on the target alert instead of briefly showing the dashboard first.
• Reuse an active PWA window when possible and mask stale iOS PWA snapshots during navigation.
• Render the alert-details loading state immediately for direct alert URLs.
• Normalize trailing slashes so /alerts/ resolves exactly like /alerts.
• Avoid duplicate syncAlertDetailsFromUrl() calls during authentication and route rendering.
• Reject cross-origin notification targets before service-worker navigation.
• Move AlertGroup API calls, OpenAPI definitions, tests, and documentation to /api/alert-groups/....
• Use /acknowledge and /{target_group_id}/merge for the final 2.3 API contract.
• Keep /alerts/<id> unchanged as the browser-facing deep-link route.

Testing

• pytest -q — 1,842 passed.
• AlertGroup API, OpenAPI, routing, PWA, modal-scroll, and notification regression tests passed.
• Ruff passed.
• JavaScript syntax validation passed for all static JavaScript files.
• git diff --check passed.

Manual verification

The mobile UX changes carried over from #90 were manually verified on iPhone:

• notification deep links open the target alert;
• alert details fit the viewport without horizontal overflow;
• the background does not scroll after reaching the end of the details view;
• the mobile alert-details theme is consistent;
• direct alert URLs load the details view correctly.

Contributor License Agreement

After opening this pull request, comment exactly:

/sign-cla

You only need to sign once per CLA version. If you already accepted the current CLA.md, the CLA status check will pass automatically.

[CoooderSan]

/sign-cla

[Aidaho12]

Thank you!