refactor(core): type the platform error boundary with PlatformError (#1985)

Replace stringly-typed Result<_, String> across the Core platform
traits, the three OS implementations, and the App services that consume
them with a structured PlatformError enum (thiserror). Variants keep the
DP-02 distinction: Unsupported (absent by design), Unavailable
(attempted, no data this time), Fault (unexpected failure), and
InitializationFailed.

- Remove the dead Core enums::error::BackendError POJO mirror; the
  network path now carries provider error messages instead of
  discarding them.
- Keep wire contracts unchanged: command signatures, generated specta
  bindings, and the frontend-facing BackendError enum stay as-is;
  commands translate typed errors at the wire boundary.
- Classify build-only stubs as Unsupported, verified by new macOS
  platform tests; cover the network wire mapping with App unit tests.

Windows/Linux compilation is verified by CI: cross-target cargo check
from macOS fails in ring's C build script (environment limitation, not
a code path).