feat: add wallet seam, discovery url params, media query hook and not found page

[Pro-builder]

Closes #133
Closes #134
Closes #135
Closes #136

Wallet seam (#133)

• WalletContext is exported from lib/stellar/WalletProvider.tsx so tests and stories can provide a fake value without loading the wallet kit.
• lib/stellar/testing/fake-wallet.ts: fakeWallet(preset, overrides) with disconnected, connecting, connected, wrongNetwork, unauthenticated and withError presets. The signer always rejects with { code: -4, message: "User declined" }, so nothing can sign.
• Tests in lib/stellar/testing/fake-wallet.test.ts.

Responsive groundwork (#134)

• lib/hooks/useMediaQuery.ts: SSR safe hook built on useSyncExternalStore, returns null on the server so prerendered HTML does not mismatch.
• Quick view close control is now a 44 px touch target (min-h-11 min-w-11).

Not found states (#135)

• New app/not-found.tsx (static) linking back to /markets.
• Quick view copy moved to sentence case: "Market not found", "Back to markets" (caps kept through CSS).

URL synced discovery (#136)

• lib/markets/discovery-params.ts: parseDiscovery and serializeDiscovery for q, status, category and sort. Defaults are omitted, keys are in a stable order, unknown values fall back to defaults, status=ALL round trips, and q is trimmed to 100 chars.
• Markets filter bar uses NETWORK_LABEL instead of the hardcoded "Stellar Testnet".
• Tests in lib/markets/discovery-params.test.ts.

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