Restore full context menu and retain FFmpeg.wasm MP3/MP4 conversion

[mrjlde]

Motivation

• A prior change unintentionally reduced the right-click menu to only the new converter items, removing the original full set of actions users relied on.
• The intent of this PR is to restore the full menu structure (Media Tools, Quick Actions, Quick Share) while keeping the newly added in-browser MP3/MP4 conversion options.
• Keep the userscript runnable in common userscript managers by preserving required grants for byte fetching and conversion (GM_xmlhttpRequest, GM_download, etc.).

Description

• Restored the larger context menu and re-added the original action groups and items (copy/open, save original, save as PNG/MP4, .URL/TXT/XML/YTF/OBX/HEX exports, quick actions, and quick share entries).
• Kept and integrated the FFmpeg.wasm conversion pipeline (ensureFFmpegLoaded, convertUrlTo, toBlobURL) and added Convert → MP3 and Convert → MP4 menu actions under a dedicated "Convert (Direct file URLs only)" section.
• Preserved right-click target detection and double-right-click quick copyAndOpen behavior, plus user-facing guardrails that reject playlist manifests and HTML page URLs before attempting conversion.

Testing

• Ran syntax check with node --check right-click-save-media.user.js, which completed successfully.
• Verified expanded UI entries by running rg -n "makeMenuItem\(" right-click-save-media.user.js | wc -l to confirm the menu items are present.
• Automated checks above passed and the updated file was committed to the branch.

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Codex Task