refactor(preview): remove signed preview URLs

[G4614]

Remove the signed preview URL token flow so port preview routing only accepts case-safe direct box IDs.

Test plan:

• git diff --check
• rg -n "SignedPortPreview|signed-preview|signedPreview|getSignedPortPreview|getBoxIdFromSignedPreview|signedPreviewToken|boxIdOrSignedToken" apps/api apps/proxy apps/dashboard apps/libs/api-client apps/api-client-go apps/infra-local || true
• go test ./pkg/proxy
• NX_DAEMON=false npx nx test api --testPathPatterns=box.service.spec.ts
• npx vitest run src/hooks/queries/useTerminalSessionQuery.test.tsx --config vite.config.mts

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