Add multi-provider secret sync

[Redth]

MAUI Sherpa currently treats cloud secret storage as a single active-provider choice, which prevents users from keeping each sensitive item in the provider set that best fits its backup and publishing needs. This change adds item-level multi-provider placement with progressive status and recoverable background reconciliation.

What changed

• Adds a provider registry and conflict-aware sync coordinator for managed secrets, Android keystores, certificates, provisioning profiles, and publish profiles.
• Uses typed item adapters and manifests so provider copies can be compared, moved, removed, and retried without persisting secret payloads in task records.
• Prioritizes Local Vault within each reconciliation and batches provider inventory reads to avoid per-row status calls.
• Adds a persistent background task center with coalescing, retries, cancellation, restart recovery, per-provider progress, live Shiny.Mediator placement events, and completed-history clearing.
• Adds a shared cloud/vault picker to every supported item surface plus per-provider, per-item-type defaults in Settings. Defaults are explicitly non-retroactive.
• Preserves the last Apple and Google identity selection and fixes native macOS toolbar labels after identity changes.
• Tightens provider not-found/error handling, including Infisical fast-path behavior, and improves macOS Debug Local Vault access and duplicate native-reference handling.

Review notes

Background records contain only item/provider references and state. They persist through Local Vault when available and fall back to memory-only execution when it is unavailable. Reconciliation is still represented as one logical item job with provider substeps; independent provider-lane scheduling is intentionally deferred so source selection, revisions, coalescing, and conflict decisions cannot diverge.

Validation

• dotnet test tests/MauiSherpa.Core.Tests/MauiSherpa.Core.Tests.csproj (552 passed)
• dotnet build src/MauiSherpa.MacOS -f net10.0-macos --no-restore --verbosity minimal