* ci: stabilize Electron dependency installs
Skip unused Electron binary downloads in non-desktop CI jobs and retry Electron installs in desktop tests to avoid external CDN 504 failures during npm ci.
* ci: preserve npm retry exit code
Preserve the final npm ci exit code in the non-Windows Electron retry path so CI diagnostics retain the original failure code.
* ci: preserve retry exit code under errexit
Keep npm ci inside the retry conditional so bash errexit does not bypass retries, while preserving the final npm exit code for diagnostics.
* fix(app): clamp split pane resize
Prevent right-edge drags from overflowing the Electron viewport.
* fix(app): isolate resize pointer state
Keep concurrent resize drags from clobbering each other.
