Harden desktop endpoint defaults and encoded pairing responses

[integry]

Objective

Track the non-blocking follow-ups from the exact-head review of remote desktop connectivity without delaying the functional/authentication merge.

Scope

• Centralize the local desktop endpoint default so the profile editor, setup wizard, examples and runtime configuration do not disagree between ports 3000 and 4000. Derive UI defaults from the shared runtime/local-setup contract.
• Make bounded desktop pairing response validation interoperable with Content-Encoding. Retain the 4 KiB decoded-body cap and all timeout/abort/redirect/UTF-8/schema checks, but do not compare a compressed Content-Length to transparently decoded Fetch bytes. Reject malformed or oversized encoded responses and add gzip/br/identity proxy fixtures.

Dependency and integration

Start after #1977 is integrated into the transport epic so the changes target the reviewed remote flow. Add the AI label and current transport descendant base label at that point. Require exact-head review and relevant client/Electron/UI tests. Never target or merge directly to main.

Follow-up to review suggestions S1 and S2 on #1977. Part of #1950, #1953, and #1961.

[integry]

Deferred from the bounded final review of remote PR #1977 (exact head 6b05bce0ef0e0ef9eea8ecfc1394828cf9846a92): preserve safe same-profile renderer preferences across renderer launches using an encrypted identity marker and/or per-profile partition. Current behavior deliberately clears in-memory renderer state on first activation after launch and is fail-closed, so this is advisory hardening and does not block integration.

[integry]

Issue/PR reconciliation: encoded pairing-response work is implemented by merged PR #2023 (issue #2022). The endpoint-default centralization item remains separately outstanding, so this issue is intentionally left open rather than closed with the already-merged partial implementation.

[integry]

Please implement only the remaining endpoint-default centralization now, targeting 1950-epic-cross-platform-dsk, never main. The encoded pairing response work is already complete in #2023; do not redo it. Same-profile renderer preference persistence remains advisory and out of this implementation.

Independent audit of epic 923d5db and setup PR #2132 a54f16b confirms DesktopExperiencePanels.tsx editor and browserAdapters.ts example still default to 127.0.0.1:3000, whereas docker/launcher/orchestrator.mjs, .env.example, and packages/local-setup/src/engine.ts use API port 4000. Establish a shared local API default contract and derive desktop editor/example/setup fallback defaults consistently. Preserve configured API ports and hosted dynamic/same-origin behavior. Coordinate with #2132: its desktopLocalSetup callback already uses actual cfg.apiPort through localhostServiceUrl; preserve that correctness and avoid unrelated wizard edits.

Keep this bounded and nonblocking for #2125/#2127/#2132 integration. Add focused regression tests, run relevant typechecks/tests, put Closes #1999 in the PR body, and request Ultrafix verification. Do not merge; report exact head and evidence.

[propr-dev]

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• Agent: codex
• Model: gpt-5.6-sol
• Branch: 1999/gpt-5.6-sol-harden-desktop-endpoint-d-20260906-1049-9n9
• Base Branch: 1950-epic-cross-platform-dsk
• Worktree: issue-1999-20260906-1049-gpt-5.6-sol-9n9

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