refactor: render original release Markdown without preprocessing

[SakuraMathcraft]

Summary

• Pass original release-note Markdown directly to the existing renderer.
• Remove unconditional mark-tag stripping and relative-image URL rewriting, including their regexes, hardcoded repository URL, helper functions, imports, and the now-unused markdown_rendering module.
• Keep existing rendering styles and empty-changelog handling. No replacement repair or fallback logic is introduced.
• Refresh translation catalog source locations only; translated text is unchanged.

Validation

• No remaining references to the removed preprocessing helpers or constants.
• Ruff passed for src/update.
• Updater tests: 6 passed.
• Translation catalog and git diff whitespace checks passed.

The previously merged DLL fix is not part of this PR.