Prepare 0.9.0: custom app identity, updater reliability and discovery fixes

[CryptoKey98]

Summary

Prepare 0.9.0 with more reliable custom GitHub application installation and detection across Windows, Linux and macOS. Verified package identities and executable choices keep actions attached to the selected installation, support ambiguous application selection, and avoid duplicate Windows choices.

• Preserve Linux desktop launcher arguments and environment settings, and retain previous identities after failed or cancelled installs.
• Fix macOS self-updates failing on unrelated symbolic links and acknowledge saved update errors once while retaining diagnostics.
• Expire New badges 14 days after repository creation for fresh and existing users.
• Include manager updates in periodic checks, preserving the existing interval and user approval for installation.
• Update version, release notes, What's New and Linux package inspection dependencies. Changes only to codemagic.yaml no longer trigger the Check workflow.

Existing package formats and filenames remain unchanged. Package-format migration is deferred. Local AGENTS.md and deferred patches are excluded.

Validation

• Windows full suite before the final periodic-check change: 315 passed, 2 fixture-dependent tests ignored.
• Final periodic-check change: 25 UI tests passed; formatting, strict all-target Clippy and Windows release build passed.
• Ubuntu discovery/expiry tests and release build passed; repository-date cache test passed.
• Earlier macOS native DMG transaction tests covered install, identity reload, reinstall, cancellation, repository mismatch and uninstall in isolated folders with security checks enabled.
• Manual Windows custom-app tests with WinMerge and CopyQ succeeded; CopyQ showed one identity choice.
• 0.9.0 release metadata validation and its five validation tests passed.

A live timed periodic-check cycle has not been exercised. Final cross-platform coverage is pending this PR's CI checks; no claim of universal third-party installer compatibility.

Do not publish a release as part of this PR.