fix: prevent staking table columns from collapsing on mobile

[sirakinb]

Fixes #81.

Summary

• Add minimum widths to the staking dashboard table columns so values do not collapse into unreadable cells on mobile-sized viewports.
• Wrap the table in a horizontal overflow container and set the mobile grid width to the sum of the column minimums, allowing touch users to scroll the table instead of seeing cramped columns.
• Keep the existing desktop flex layout behavior at md and above.

Testing

• Not run locally: this environment does not have npm/yarn/pnpm available and the fresh clone has no node_modules.

[sirakinb]

Small CI note: this PR is hitting the same CLA Assistant infrastructure failure as the depeg-ui PR.

The annotation is:

Could not retrieve repository contents. Status: 401

The workflow reads signatures from etherisc/cla-signatures via PERSONAL_ACCESS_TOKEN, so this looks like a workflow credential/access issue rather than a code/test failure in this PR. Happy to complete the CLA step once the workflow can prompt/check it.

[sirakinb]

Maintainer note: the CLA Assistant check is failing before it can evaluate the PR. The workflow reports that it could not retrieve repository contents with status 401, which looks like a CLA Assistant token/app permission issue on the repository side rather than a code failure in this PR.