fix(services): explain private Browser Engine requests

[andersalm]

A signed request for a private Browser Engine reaches Inbox with the instruction to share it in Services before approval. The owner can enable Share and approve the same pending request. Services explains those steps while the requester waits.

Runtime retains the signed request, accepted contact, current sharing, owner authority and scoped execution grant checks. Private model catalogue responses remain conditional on model sharing. The reviewed mailbox assertion from #145 / PR #146 is integrated.

Accepted validation covers four focused authority tests, the four basic gates, Browser entropy and Services UI smoke. Desktop and narrow captures were inspected, and independent security review passed. All eight checks passed at source 26a891c in CI run 36786102353.

This PR is integrated in develop at de1db92. Irzhy's post-merge review is required because it touches Inbox approvals. Any finding must be fixed or reverted before Tuesday's candidate. His existing review request is retained. Full installed Browser/Wallet journeys and human product acceptance remain open in #87. Refs #87 and #84.

[andersalm]

Operator note (Anders's assistant): this PR needs @irzhywau's review before it merges. It changes the Inbox approval path: home_services_record_access_decision gets a new refusal branch, and home_services_sync_access_requests now syncs peer access requests even when this Home shares nothing, which turns them into Inbox notifications. Under the weekly rules (PR #151), a PR that touches Inbox approvals has Irzhy as a required reviewer, so green checks and the agent review are not enough here. Please also answer in the PR whether a peer can now fill the Inbox with requests for a private Engine, and what limits that.

[andersalm]

Operator note (Anders's assistant): this merged at 23:30 UTC on an earlier instruction of mine that listed #147 for merge on green. That instruction was wrong, because the PR touches Inbox approvals. @irzhywau, please review it after merge. If you find a problem, it is fixed or reverted on develop before the 0.7.2 candidate is cut on Tuesday.