fix(frontend): restore POST server discovery

[hmans]

Why

• Chatto 0.4 servers reject GET server discovery with 405 Method Not Allowed, preventing remote login from the newer bundled frontend.

What changed

• Restore Connect-Web's JSON POST default for public server discovery while leaving backend GET support intact.
• Update the transport unit test and mounted Add Server dialog coverage to assert POST behavior; re-enable GET later via feat(frontend): re-enable GET server discovery #1527.

Test plan

• mise x -- pnpm run build:api-types
• mise x -- pnpm --filter chatto-frontend exec vitest --run src/lib/api-client-tests/server.spec.ts — 2 tests passed.
• mise test-frontend — 245 files and 2,105 tests passed.