feat(app): enable the supported relay by default

[frahlg]

What changes

• Treat an omitted app_link section as enabled, so upgraded boxes use the supported FTW app relay without a YAML edit.
• Preserve app_link.enabled: false and an empty app_link: {} as opt-outs.
• Keep the Settings fallback in step with Core and make the example config show the default.
• State the privacy boundary plainly: relay frames are end-to-end encrypted, but the relay still sees source IP, timing, and connection continuity.
• Update the app-uplink ADR to match the shipped pairing, device-removal, and owner rules.

The relay endpoint and protocol remain fixed. This change does not alter pairing keys, command checks, or hardware control.

Compatibility proof

The new round-trip test starts with an old YAML file that has no app_link, loads it, fetches it through GET /api/config, saves that response through POST /api/config, reads the file again, and proves the app link stays on after restart. Separate tests keep explicit false and an empty section off.

Verification

• make verify
• go test -race ./internal/config ./cmd/ftw ./internal/appuplink ./internal/appenroll ./internal/api
• npm test — 339 passed

Release order

Merge this before the generated Version Packages PR #824. Changesets must then refresh #824 so the corrected privacy text and the default-on behavior appear in the 2.0.0 changelog. Do not merge #824 from its current head.

The Settings change still needs a human browser check on mobile and desktop during the beta: omitted config, explicit opt-out, toggle/restart copy, status, pairing, and QR layout.

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[frahlg]

@miravoss26 could you review exact head 16cecec0? It enables the supported relay by default while preserving explicit YAML null/false opt-outs. Focused tests, race, full make verify, full-stack CI, and all required GitHub checks are green. The one automated review finding is fixed and resolved.