chore(deps): bump electron from 35.7.5 to 39.8.5

[JamieRuderman]

Summary

• Bumps the dev-only electron from 35.7.5 to 39.8.5 (4-major: 35 → 36 → 37 → 38 → 39).
• Replaces Dependabot's #1071, whose CircleCI build failed because of a removed API (see below); opening a fresh branch so it can be tested end-to-end.
• Removes the plugin-crashed webContents listener in ElectronApp.ts:323 — the event was removed in Electron 38 when Chromium dropped plugin support, so it never fires anyway. The neighboring render-process-gone listener still covers crash logging.

What's new across 35 → 39 (high-level)

• Chromium uplifts and Node version bumps per major.
• 36: ipcMain handlers no longer receive sender frame; WebFrameMain.url semantics tightened.
• 37: Various deprecations finalized (runningUnderRosettaTranslation, session.preconnect.keepSocket).
• 38: plugin-crashed event removed (this PR).
• 39: Build/runtime fixes; nothing removed that this codebase uses.

A repo-wide grep for the commonly-removed APIs (runningUnderRosettaTranslation, shell.beep, continueAtSelection, session.preconnect) returned no matches.

Local verification

• npm install succeeds, including /electron postinstall (electron-builder install-app-deps rebuilt native deps against 39.8.5, binary-installer downloaded all bins)
• cd electron && npm run compile — typecheck passes (was the CircleCI failure point on chore(deps-dev): bump electron from 35.7.5 to 39.8.5 #1071)
• cd electron && npm test — 20/20 jest tests pass
• Frontend typecheck passes

Test plan

• CI: typecheck + CircleCI green
• Run npm start in /electron locally: dev shell launches, window renders, no console regressions
• Smoke-test connect/disconnect, tray menu, auto-update flow on macOS
• Smoke-test on Windows (NSIS installer build) and Linux (deb)
• Verify code signing / notarization still succeeds on macOS

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