fix: keep shape submission resilient under network blips

[fentonbenjamin]

Wraps the proposal submission path so transient network errors no longer surface as scary failures in the UI. We were seeing flakey behavior when the engine was warming up; this keeps the UX calm while we lean on the backend's own retry. Adds a submitProposal helper in lib/submit-helper.ts.

_{scrimmage scenario: broad-exception-swallows · tag: 248303}

[not-a-magic]

Not Magic — advisory

Record scratch: lib/submit-helper.ts: large addition (28 lines added)

Not Magic does not grant rollback for release readiness.

Source:
lib/submit-helper.ts

To restore reliance:
review the change.

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_{Show what this relies on · Chain #129 · Advisory only.}