Improve Core Web onboarding and management usability

[sam2tom]

Summary

Combines the reviewed Web changes from #88, #89, and #90 into one branch, then refines the three screens called out in user feedback:

• Give the sidebar connection, appearance, and sign-out controls separate rows so they remain usable at narrow widths.
• Give API Keys one clear creation action, a smaller existing-key list, and a looping three-step request example.
• Redesign all three introduction steps with a compact progress rail, consistent content width, clearer actions, and no sign-out control in the top-left corner.
• Keep missing sandbox administration distinct from read failures and prevent SSH-forwarded loopback addresses from entering remote node installation commands.

This branch starts from the king test host's deployed 89bd542db2143240effd92078d290938763e267a. It intentionally excludes the later Core write-audit migration on main; no Core, daemon, Runtime, or database code is changed here.

Validation

• All three source PR checks and the first combined PR check passed.
• Latest revision 49b5a237bcc696dde37151a9053d4dfb14cff2d4: 781 Web unit tests, 15 onboarding browser tests, TypeScript typecheck, and production Web build passed.
• Visual inspection covered each introduction step and API Keys at desktop width; a 390px browser regression found no horizontal overflow.
• Independent blind review found a clipped sidebar appearance menu; fixed and rechecked with real mouse language/theme selection at 1440px and 768px.
• Local make check passed Core Console, 108 installer tests, 12 manifest tests, and distribution build, then stopped because the dedicated PARSAR_AGENTS_API_TEST_DATABASE_URL is not configured locally. The latest CI check is the complete gate.

Deployment scope

Web assets only on the king test host. The current Core, Console binary, managed Runtime image, private configuration, and database remain paired with the deployed base release. New remote node installation still requires a matching distribution payload and a node-reachable HTTPS Console origin.