feat: rework local builds

[codebytere]

Closes #1833
Closes #1366

The way local builds work right now is pretty broken — they get dumped into the same versions dropdown as remote releases with a user-provided semver string as the identifier. If someone gives their local build the same version as an existing release (e.g. v40.0.0), things go sideways.

This reworks local builds to be properly separated from remote versions:

• Local builds now get a user-friendly name instead of requiring a semver version string. The internal version key is auto-generated (0.0.0-local.<timestamp>) so it can never collide with a real release.
• The version dropdown splits local builds and releases into separate sections with headers.
• The settings page does the same — local builds and releases are visually separated with color-coded section headers and row accents (green for local, blue for remote).
• Local builds always show up in versionsToShow regardless of channel/obsolete/download filters.
• The "Electron v" prefix in the toolbar is suppressed for local builds, and the right-click "Copy Version Number" context menu is hidden since the internal key isn't meaningful.

Screenshots

[coveralls]

coverage: 79.622% (+0.08%) from 79.539%
when pulling e6a8132 on improve-local-builds
into 1ec6283 on main.