feat: add chain comparison matrix page

[blazesgl]

Summary

This PR adds a dedicated /chains route with a comparison matrix that lets prospective integrators compare supported blockchain networks side-by-side.

Changes

• Added src/data/chains.json with chain comparison data and per-chain descriptions
• Created src/pages/Chains.tsx with sortable columns, status filter, expandable row detail, horizontal scroll on mobile
• Updated src/App.tsx with /chains route
• Updated src/data/README.md with chains.json schema documentation

Acceptance Criteria

• /chains route added
• Matrix sorts on every numeric column (block time, median fee)
• Every row has a documented data source in the JSON
• Matrix renders without CLS on mobile (horizontal scroll container)
• Build passes
• Lighthouse Perf/SEO stay >= 90

Closes #127

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[truthixify]

Merged @blazesgl. Sortable matrix, a documented source per row, and horizontal scroll so it does not blow up the layout on mobile. Good work.

One thing to flag for whoever picks it up, not a problem with this PR: we now have two chain data sources in the repo.

• src/data/chains.json (this PR) with id, name, blockTime, medianFee, finality, wallets, status, audit, docs, description
• src/data/calculatorChains.ts (from feat: add stealth payment cost calculator #147, the cost calculator) with id, name, networkFeeUsd, sourceLabel, sourceUrl

That is my doing. When I assigned the calculator I told @ranvirjrj-beep not to block on this page and to stub their own data, so the overlap was expected. The ids match exactly (stellar, solana, nervos-ckb, horizen), and the only real conflict is medianFee versus networkFeeUsd describing the same number.

The tidy end state is chains.json absorbing networkFeeUsd, sourceLabel and sourceUrl, and CostCalculator reading from it, so a fee is corrected in one place rather than two. Worth noting the calculator's acceptance required inline source attribution, so those two fields have to survive the merge.