Automate releases and add silent background updates

[Karl-Dai]

What changed

• align the automated nightly, merge, and tagged release workflows with the IEC104 release pipeline
• download and signature-verify updates silently before showing the dialog
• offer install now, skip this version, or install automatically on next launch
• apply the same state machine to both Modbus slave and master applications

Why

The previous updater waited for the user before downloading and only supported a temporary snooze. The new flow keeps startup unobtrusive and only interrupts the user once an update is ready to install.

Validation

• cargo check -p modbussim-app -p modbusmaster-app
• cargo test -p modbussim-app --test update_helpers
• cargo test -p modbusmaster-app --test update_helpers
• production builds for frontend and master-frontend
• git diff --check