feat(desktop): add backend selector to edit and import

[cameronhotchkies]

Why

Editing an existing managed agent or importing a team snapshot always persisted local execution, forcing users to delete and recreate agents to run them on Blox or another provider.

What

• Add the shared Run on selector to instance edit and team snapshot import confirm flows
• Thread selected backends through TS APIs into Rust update/import request types
• Apply backend changes in Rust, including provider config validation, binary resolution, and deploy calls outside the store lock
• Add regression tests for absent/update/import backend default behavior

Risk Assessment

Medium — this touches Desktop agent persistence and provider deploy paths, but defaults preserve existing local behavior and backend updates are opt-in.

References

Validation:

• cargo test --manifest-path desktop/src-tauri/Cargo.toml
• pnpm --dir desktop check
• pnpm --dir desktop typecheck

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