fix: prevent duplicate unstake submissions

[sirakinb]

Fixes #204.

This prevents a fast double-click on the Unstake submit button from starting duplicate unstake transactions:

• adds an immediate ref-based in-flight guard at the form submit boundary
• disables the submit button while the unstake action is in progress
• preserves the existing parent transaction flow and step handling
• adds a regression test that double-clicks the Unstake button and asserts only one unstake call is made

Testing:

• git diff --check
• npm run lint -- --file src/components/unstake/unstake_bundle.tsx --file src/components/unstake/unstake_bundle_form.tsx
• npm test -- --runInBand tests/components/unstake/unstake_bundle_form.test.tsx

[sirakinb]

Note on the failing CLA check: GitGuardian has passed, and the failing job is only CLAAssistant.

This looks like the same repository-side CLA Assistant infrastructure issue currently affecting the other Etherisc PRs: the CLA job itself fails, while the code/security check is green. Could a maintainer please rerun/fix the CLA Assistant token or advise how you want contributor CLA handled here?