Modernize browser demo models with supported NanoGPT choices

[mikkel]

The browser demos used factory model IDs that are no longer advertised in NanoGPT's current catalog. Use xiaomi/mimo-v2.6-flash for diff generation and scoped repairs, and pin MiMo 2.6 Pro and GLM 5.3 Flash as inexpensive alternatives. Existing factory selections migrate; custom saved choices remain intact.

The decision demo keeps MiMo 2.6 Flash and requests its supported none reasoning effort. Preserve catalog creation dates so the picker can rank each lab's newer flagship correctly. Public catalog excerpts record the IDs, capabilities, and prices verified on 2026-10-13.

Validation:

• Full offline JS suite: 175 passed, 4 opt-in live tests skipped.
• New mocked model/SSE regressions: 20 passed across both repositories on Node 18.20.8.
• Every inline demo script parsed successfully.
• Deterministic benchmark: 112 clean-correct file patches, 23 requiring fallback, zero silent wrong applications.
• No paid inference was run; model response quality and live latency were not measured.

This changes demo pages, documentation, and tests. Library defaults, package versions, release checks, and UTF-8/binary handling are unchanged. The existing CI release-test failure is reproducible in a shallow checkout without tags: package 0.3.0 has no unreleased entry when no release tags are visible. This PR deliberately does not include that separate release fix.