feat: in-app auto-update

[VitorRibeiroCustodio]

What

In-app auto-update — users move to a newer version without deleting and reinstalling the app (#91).

• Silent update check ~10s after launch; manual Check for updates in Settings → Diagnostics → Updates.
• A sidebar header button appears whenever an update is available / downloading / downloaded — the single apply path: one click downloads, then becomes restart & install. Hidden when up to date, unsupported, or the last check failed.
• The Settings Updates section shows the current version, a manual check, and the last-check outcome (up to date / available / error). It no longer carries its own apply buttons — applying happens via the sidebar button, so there is one apply path.
• Auto-update is a packaged-build capability. Dev runs and the Linux deb target (no in-place update channel) report "unavailable" instead of failing.
• Once downloaded, Restart & install relaunches onto the new version; a deferred install also applies automatically on next quit.

How

• New backend module electron/ipc/updater.ts wraps electron-updater's autoUpdater behind new IPC channels: check_for_updates, download_update, quit_and_install_update, get_update_status, and a pushed update_status_changed event.
• Renderer store slice src/store/updates.ts (status signal + subscription + check/download/install helpers); new UpdateButton component in the Sidebar header; Updates section in SettingsDialog.
• electron-builder config: GitHub publish target + a macOS zip artifact (electron-updater requires zip to apply macOS updates).
• No persisted-state or schema change — update status is transient, re-derived from the main process each launch.

Testing

Verified locally against a generic update server: a packaged build detected a newer published version, downloaded it with progress, and relaunched onto the new version via Restart & install.

Screenshots

[johannesjo]

Thank you very much! <3