fix(core): resolve repository quality audit findings

[StudentWeis]

Summary

Resolves every actionable finding from the repository-wide code, comment, and test-quality audit. The changes harden clipboard writes, platform activation, repository deletion, updater integrity, settings persistence, localization, documentation, and test isolation.

Linked Issue

Closes #157

Changes

• Propagate clipboard write failures, distinguish rich-text payloads, and hash complete image identity.
• Make repository deletion/cleanup/clear atomic across trees and defer sidecar removal until commit succeeds.
• Remove unbounded X11 activation polling and identify the Ropy window specifically on Windows.
• Require valid update checksums in manifests and downloads; save settings atomically without overwriting malformed configuration.
• Isolate settings/autostart tests, localize file labels, and enforce warning-free Rustdoc in precheck.

Testing

• scripts/precheck.sh passes locally
• New / updated tests cover the change
• 471 tests pass
• RUSTDOCFLAGS="-D warnings" cargo doc --no-deps passes
• Update manifest generation rejects missing checksums and accepts complete archive/checksum pairs

Self-Check

• PR title follows Conventional Commits
• No hardcoded user-facing strings — i18n keys added to all locale files
• Errors defined with thiserror (no manual impl Display/Error)
• No unrelated changes mixed in