feat: add stealth payment cost calculator

[ranvirjrj-beep]

Summary

• Add a shareable stealth-payment cost calculator with chain, monthly payment count, and average payment value inputs.
• Embed it below the /use-cases personas and add /use-cases/calculator.
• Persist scenario state in URL query parameters with copy/reset, validation, inline source notes, and accessibility coverage.
• Keep fee data behind an explicitly temporary typed adapter while Chain comparison matrix page #127 owns canonical src/data/chains.json.

Verification coverage

• 10 focused calculator regression tests
• unit axe coverage
• Playwright axe + keyboard coverage
• sitemap and SEO metadata coverage

Data dependency

#127 is still open and src/data/chains.json has not landed. Per the maintainer's instruction on #128, this PR uses an explicitly noncanonical temporary adapter designed to be replaced by the actual #127 schema without rewriting calculator logic.

Closes #128

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[truthixify]

Merged. Stubbing calculatorChains.ts locally rather than blocking on #127 was exactly right, and the URL-encoded scenario state plus the a11y test make it genuinely shareable. Good work @ranvirjrj-beep.

[ranvirjrj-beep]

@truthixify Thank you! Really appreciate the review and the kind words. If possible, I’d also be grateful if you could leave feedback for me on Stellar. I’d be very happy to take on larger or more involved issues as well, so please keep me in mind for future work. 🙌