feat(storybook): add Storybook 8 with stories for Stellar components

[Kingvic300]

Set up Storybook 8 (Vite + React) wired to the dark monochrome design system, with stories for the key Stellar components and a test-runner visual-regression job.

• Split Stellar components into pure views + containers: StellarSendView, StellarReceiveView, StellarMatchCard, FreighterConnectButton; add a disabledChains prop to ChainSwitcher
• Export ChainContext/StealthKeysContext/StellarWalletContext so stories can inject mocked values
• Add .storybook config (dark theme, fonts, MSW worker) and mock-context decorators (withChain, withStealthKeys, withStellarWallet) + fixtures
• Stories for CopyButton, ChainSwitcher, FreighterConnectButton, StellarMatchCard, StellarSendView, StellarReceiveView, plus WalletConnect and StellarReceive container integration stories, with idle/loading/error variants and no network or wallet access
• Visual regression via @storybook/test-runner + jest-image-snapshot, CI workflow running inside the Playwright image, and CONTRIBUTING docs

Closes: #25

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[truthixify]

The Storybook setup, the stories themselves, and the new presentational StellarSendView.tsx extraction look like a solid foundation for #25. Conflicts with develop after recent merges, please rebase:

git fetch origin
git rebase origin/develop
git push --force-with-lease

When resolving, keep develop's StellarSend.tsx work (validation, sponsored withdrawal, retry recipes) and rebuild your StellarSendView.tsx extraction on top of that rather than reverting to the pre-validation version. Thanks @Kingvic300.

[Kingvic300]

@truthixify this has been fixed

[truthixify]

Verified locally that the merge cleanly preserves #13's validation, #29's sponsored withdrawal, and #31's faq.ts. The presentational StellarSendView / StellarReceiveView extraction is the right shape for Storybook. Merging. Thanks @Kingvic300.