chore(deps): bump electron 41.5.0 -> 42.1.0 and pin packaged version (#9959)

Bumps the desktop runtime to electron 42 (dependabot PR #9945) and
closes a supply-chain gap in the Linux/Mac packaging scripts that
predated this bump.

Why the bump is safe:

  - macOS UNNotification API change — pgAdmin's runtime does not use
    Electron's Notification API (only a UI toast comment in
    src/js/pgadmin.js:211; no `new Notification(...)` anywhere).
  - postinstall no longer downloads electron — production packaging
    fetches the binary directly via wget from GitHub releases, never
    via electron's postinstall script.
  - Offscreen rendering scale-factor change — no OSR usage anywhere
    in runtime/src/js/.

While verifying, found that pkg/linux/build-functions.sh and
pkg/mac/build-functions.sh resolve the packaged electron version
via:

    ELECTRON_VERSION="$(npm info electron version)"

This pulls whatever currently carries the `latest` dist-tag on the
npm registry. Any newly published electron release — including a
hypothetical malicious one — would land in shipped binaries without
review, regardless of what runtime/package.json pins.

Replace with sed-based extraction from runtime/package.json and
fail loudly if extraction returns empty. The Windows installer
(pkg/win32/installer.iss.in) does not have this issue (it bundles a
pre-built tree, no electron download step).

Net change in runtime/yarn.lock is mostly deletions — electron 42
ships with @electron/get 5.x, which dropped a large transitive
dependency tree associated with the old postinstall download path.

Verified:

  - eslint (runtime): clean (silent)
  - yarn install (runtime): resolved to electron 42.2.0 within
    ^42.1.0 range
  - sed extraction smoke-tested: returns 42.1.0 from current
    runtime/package.json