fix(update): improve download reliability and progress reporting

[marcomarcogd]

Summary

• reuse the proxy-aware update HTTP client for downloads
• report real byte progress instead of simulated percentages
• retry transient network and 5xx failures with bounded backoff
• resume interrupted downloads with Range and restart safely when unsupported
• remove incomplete files and keep installation disabled after failure
• expose short network-specific errors and a manual release-page fallback

This improves reliability but does not bypass networks that cannot reach GitHub and have no usable proxy.

Fixes #1043

Validation

• go test -timeout=5m ./...
• frontend Vitest: 81/81
• auto-update Cypress: 14/14
• frontend production build
• target ESLint
• git diff --check