feat(update): default checks and installers to CDN

[debidong]

Summary

• default update checks to the Flashcat CDN latest pointer and keep GitHub release page URLs for display
• make first daily auto update checks synchronous with a short timeout warning path
• switch installer defaults, docs, and release workflow rewrites to CDN public URLs
• harden installer execution by passing installer URLs as process args instead of interpolating shell commands

Test plan

• go test ./...
• git diff --check
• sh -n install.sh
• bash -n install.sh
• curl -fsSL --max-time 5 https://static.flashcat.cloud/flashduty-cli/releases/latest
• curl -fsSI --max-time 5 https://static.flashcat.cloud/flashduty-cli/install.sh
• curl -fsSI --max-time 5 https://static.flashcat.cloud/flashduty-cli/install.ps1

Notes

• Local pwsh and shellcheck are not installed, so PowerShell/script lint was not run locally.
• Existing untracked local file flashduty was intentionally left out of the commit.