Add Download Center and make OS/App/Files directly downloadable in core pages

[Rtech-Rqbbox-os]

Motivation

• Provide clear, immediate download links for the RQBBOX OS image, ZIP package, app entry, and full project bundle from the core UI so users can obtain assets without digging through the repo.
• Improve the in-OS File Manager and landing pages so listed files are actionable (open/download) instead of static placeholders.

Description

• Added a new "Download Center" panel to usb-software/core/index.html with links to the QCOW2 image, ZIP package, app launch, and full repo ZIP.
• Expanded CTA area in usb-software/core/index-v2.html to include explicit download/open actions for the app, QCOW2, OS ZIP, and repo ZIP.
• Reworked File Manager rendering in usb-software/core/os-shell.html to show clickable download/open links for the OS image, ZIP, app, repo ZIP, scripts, config, and README.
• Applied the same File Manager improvements to usb-software/core/os-shell-v2.html and updated displayed labels to indicate the entries are downloadable.

Testing

• Confirmed assets and repository files are present using rg --files and inspected the updated pages with nl -ba/sed -n to verify the new markup and links appear in the modified files; these checks succeeded.
• Opened the edited HTML/JS fragments with sed to validate the new download link URLs and file-item markup are correctly inserted; validation succeeded.
• Verified the working tree shows the four core files modified and that their diffs contain the intended changes by inspecting the file outputs; inspection succeeded.

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Codex Task