Hybrid/caBLE: delegate incoming FIDO:/ requests to other Credential Manager providers

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**Is your feature request related to a problem? Please describe.**
Since 0.3.14 microG handles incoming FIDO:/ hybrid (caBLE) requests correctly — the tunnel is established and the flow completes. However, the credential is only sourced from microG's own passkey store.

Passkeys stored in a third-party Credential Manager provider (tested with Proton Pass, set as the system's default passkey provider) are never offered. This makes cross-device authentication unusable for users who keep their passkeys in a syncing password manager, which is arguably the main reason to have a third-party provider at all.

**Describe the solution you'd like**
After the hybrid handshake, query the Credential Manager providers the user has enabled instead of only microG's internal store — ideally via the normal provider selection UI, so any provider works without
per-app support.

**Describe alternatives you've considered**
A clear and concise description of any alternative solutions or features you've considered.

**Additional context**
- This mirrors the limitation of Google Play Services on stock Android; 1Password, Bitwarden and Proton Pass users hit the same wall there. microG is in a position to do better.
- Possible complication: the request is for a foreign relying party, so setOrigin() semantics apply. Per Android's "privileged apps" docs the provider must allowlist the calling app, which may need coordination with provider vendors.
- Related: #2150 (hybrid transport itself, implemented via #3194), #3290 (missing intent handler pre-0.3.14).

Start by tracing the existing hybrid transport and handshake work from #2150 and #3194, then inspect the intent-handler path added for #3290 and Android Credential Manager provider-selection and setOrigin behavior. Done means an incoming FIDO:/ request can offer enabled third-party providers, including the foreign relying-party case, with provider allowlisting constraints addressed.

Written by the indexing model from the issue text.