feat(storybook): begin the approved Gate C UI foundation

[dDevAhmed]

Begins #552 as a stacked draft on #562.

Initial foundation

• normalizes Storybook 10 / Next.js Vite configuration;
• binds stories to the production semantic-token stylesheet;
• adds light/dark authority controls and the approved 390/768/1440 responsive viewports;
• makes accessibility violations fail Storybook checks;
• adds a named Gate C manager surface;
• documents fixture, authority, state, runtime and production-boundary rules;
• adds a required Storybook build job to the repaired frontend CI matrix.

Why this is stacked

#552 requires a verified frontend baseline. This PR targets stabilization/fe-ci-baseline-558 so it cannot be merged independently of #562.

Remaining before #552 can close

• remove or quarantine legacy template/example stories;
• map every retained component story to the merged protocol UX authority;
• add the complete loading/empty/error/stale/disabled and transaction-state matrix;
• add interaction/a11y/visual-regression execution, not only a static catalog build;
• prove Storybook fixtures and synthetic data are excluded from production bundles;
• obtain exact-head CI evidence and independent human approval.

No wallet, API or contract integration is introduced, and no Stellar Wave label is applied.

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