feat: discover models from AI providers

[Bihrys]

Summary

• discover models from OpenAI-compatible GET /v1/models endpoints
• authenticate discovery requests with the provider API key
• deduplicate and sort returned model IDs
• automatically discover models when saving a provider and persist the full list
• add a refresh action and model picker to the provider editor
• retain manual model entry as a fallback for providers without model discovery
• add English, Simplified Chinese, and Traditional Chinese labels

Validation

• endpoint construction, authorization header, parsing, sorting, and deduplication verified with a fake HTTP adapter
• targeted Dart analysis has no new issues (one existing agent_page.dart warning remains)
• translation JSON files parse successfully as UTF-8
• full Flutter tests are currently blocked on Windows by the existing tailscale 0.8.0 native build referencing POSIX-only symbols

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[LittleSheep2Code]

The CI complaint about the warnings, please fix those in order to get merged.