Fix peer visibility HTTP method compatibility

[stef-k]

Summary

• align the mobile peer-visibility request with Wayfarer's authoritative POST endpoint
• preserve the existing authenticated route, boolean payload, cancellation, and bounded failure behavior
• prove the request contract against production GroupsService with deterministic awaited capture

Closes #254

Product behavior

The Groups peer-visibility toggle now sends exactly one authenticated request using:

POST /api/mobile/groups/{groupId}/peer-visibility

The existing { "disabled": true|false } body and UI success/failure behavior are unchanged. No dual-verb retry, compatibility probe, fallback, backend change, or state redesign is included.

Validation

• production GroupsService tests: 3 passed on three consecutive runs
• relevant Groups view-model selection: 69 passed
• complete Release suite: 1,889 passed, 0 failed, 0 skipped
• Release test/Core build: passed, 0 errors
• forced compilation: 9 retained unrelated warnings, 0 errors
• Code Guard: changed guards passed; one retained unrelated parser-depth REVIEW
• git diff --check 07524b34...HEAD: passed
• no real backend/provider contact

Test ownership

The focused tests source-link and instantiate the actual production GroupsService. Request content is captured inside one awaited SendAsync return delegate, so assertions cannot race request disposal or unawaited callbacks.

Migration and API notes

• no schema or migration changes
• no change to authentication generally, synchronization, Trips, visits, SSE, routing, or offline data
• dormant user-location SSE cleanup is separately tracked by Remove dormant user-location SSE subscription seam #256

UI evidence

No screenshot is needed: the existing toggle and user-facing behavior are unchanged; this corrects its internal HTTP verb.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
WayfarerMobile.Core	87%	79%	1000	✔
Summary	87% (1381 / 1579)	79% (691 / 874)	1000	✔