feat: add UnoRouter as a chat completion source

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Checklist:

• I have read the Contribution guidelines.

Summary

Adds UnoRouter as a Chat Completion source. UnoRouter is a fully open-source OpenAI-compatible gateway (github.com/unorouter) serving 200+ models, most with a genuine free tier (:free suffix, no card needed) - which makes it a natural fit for SillyTavern users.

Follows the shape of the recent MiniMax (#5452) and AI/ML API (#4135) additions, implemented against current staging conventions (generic settingsToUpdate load/save, generic OpenAI-compatible backend path):

• src/constants.js + src/endpoints/secrets.js: source + secret key
• src/endpoints/backends/chat-completions.js: /status (dynamic /v1/models list) and /generate branches through the generic OpenAI-compatible path (streaming, tools, json_schema inherited); reasoning_effort passed through
• public/scripts/openai.js: settings binding, dynamic model dropdown (aggregator-style, no hardcoded catalog; default deepseek-v4-flash:free), reasoning effort + reasoning_content streaming extraction, max-context via the unlocked/128k aggregator pattern
• public/index.html: source option + API key form (key link: https://unorouter.com/token) + added to the data-source feature blocks matching CometAPI capabilities
• Icon public/img/unorouter.svg (own logo, currentColor)
• Kept lean: no tokenizer entry (aggregator default estimation), no SD/caption extension wiring, no endpoint selector (fixed URL)

Diff: 11 files, +95/-11.

Testing

Real end-to-end against this branch (node server.js, clean default-user):

1. API Connections -> Chat Completion -> UnoRouter, pasted key, Connect -> status "Valid", model dropdown populated with 335 models fetched live from /v1/models.
2. Selected step-3.7-flash:free, sent a message in the default chat -> streamed assistant reply received.
3. npm run lint clean on all changed files; node --check passes on backend files.

[0-don]

Companion docs PR: SillyTavern/SillyTavern-Docs#210 (UnoRouter section on the Chat Completions page, Electron Hub format).