fix(sdk): update default serverUrl to official mainnet relayer endpoint

[ducnmm]

Resolves #657

Summary

Updates the default serverUrl in the TypeScript SDK (MemWal and MemWalManual) from legacy https://relayer.memwal.ai/ to the official production endpoint https://relayer.memory.walrus.xyz.

Changes

• Updated fallback serverUrl in packages/sdk/src/memwal.ts and packages/sdk/src/manual.ts.
• Updated TSDoc comments and type definitions in packages/sdk/src/types.ts.
• Updated environment variable documentation in docs/reference/environment-variables.md.
• Updated mismatch detection in apps/app/src/pages/Dashboard.tsx.
• Added unit test packages/sdk/test/default-server-url.test.mjs to assert default serverUrl behavior for MemWal and MemWalManual.

Verification

• pnpm --filter @mysten-incubation/memwal test (33/33 tests pass)
• node scripts/check-docs-freshness.mjs (passes)

[jessiemongeon1]

Style Guide Audit

All 1 file(s) pass the style guide audit.

[HoangDucBach]

LGTM ✅

NIT: I'm a bit worried the default host is hardcoded in ~5 places across sdk, mcp and status. Every domain change means touching all of them again, which is roughly how this issue came up. If there's an intentional reason to keep them separate that's fine, otherwise I think we should extract a shared constant.