ai-config: provider-connections + model-selection UI (per-task defaults)

[mkotelnikov]

M1a code work. Implements product tickets sandclaw-ai/internal.sandclaw.ai#18 (P0 chat blockers) + #17 (per-task model config) in ai-config.core / ai-config.view.react:

• provider connections (direct + OpenAI-compatible proxies; drop Anthropic — CORS)
• model selection by type (chat/summarize/embed) + origin; bundled defaults with retired-model guards; live catalog + mark retired
• fix P0 session-unlock predicate for OpenAI-compatible connections; fail-never-silently + classified errors + novice API-key on-ramp
  Epic:

[mkotelnikov]

Part of M1a epic: https://github.com/sandclaw-ai/internal.sandclaw.ai/issues/24