feat(desktop-main): electron-updater wired with disabled feature flag

[CoderCoco]

Child of #141 (epic: distribution + auto-update scaffolding). Authoritative scope in the design spec.

Scope

Add electron-updater to the main process initialization. Behind an enableAutoUpdate boolean in electron-store, default false. When false, the updater is constructed but never calls checkForUpdates. Telemetry around update checks is gated by the same flag — no silent network calls.

Acceptance

• First launch produces zero outbound traffic to update servers.
• Flipping the flag (manual test only) shows update prompts as expected once we have a signed build.