feat: add Vietnamese localization support

[tinyc0der]

This PR adds full Vietnamese localization support, including:

• UI message bundles (vi.json)
• Session metadata (titles, subtitles, key insights)
• Architecture annotation translations
• Documentation extraction support for Vietnamese content

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[Bill-Billion]

Thanks for the contribution. I’m going to close this for now because adding another locale would add ongoing synchronization and review work to this teaching repo, and we are currently keeping the main repository focused on the existing maintained languages and chapter structure.

This is not a judgment on the value of the translation itself. You are very welcome to continue maintaining this localization in your own fork, and I’m happy for people who need that language version to use it there.