[ivorb]

Stacked on #1 (base is that branch; retarget to main after #1 merges).

What

tools/translate.mjs translates a deck's strings.json into strings.<lang>.json, following docs/string-table.md. Built in a separate session, rebased onto the converter branch (the only conflict was README), and extended here.

• New copilot provider, now the default. Runs the GitHub Copilot CLI non-interactively, so translation uses the models included with a Copilot subscription: no API key, nothing to configure beyond signing in once. Tools, MCP servers and custom instructions are disabled, so the model can only answer; the prompt goes in on stdin.
• Generic OpenAI-compatible provider and an offline mock provider stay available. GitHub Models was retired on 2026-07-30, so it is not offered.
• Incremental (only new or changed strings), validated (tags and hrefs must survive; one retry, then skip and fall back to source), exit code 2 if anything was skipped.
• Adds strings.de.json for the 5-slide sample deck so deck.html?d=sample-ai-apps&lang=de works.

Checked

• node --test: 37 pass (converter tests + 26 translate tests + 2 new provider tests, no network).
• Mock run on the sample deck: 120 strings, second run keeps all 120.
• Live run via the Copilot CLI on the sample deck into German: 120/120 translated, 0 skipped, about 45 s. Spot-checked quality (idioms, alt text, copyright line) and viewed the result in the deck.

Not done

Only German was tried live; RTL languages and large decks (about 1,000 strings) are untested live. Batches run sequentially.