Wire @crema/llm-providers-ui: multi-provider picker + AI persistence

Replaces the single-base-URL LLM settings with the new providers lib
(OpenAI, Anthropic, DeepSeek, Qwen, LM Studio). Settings/LLM hosts the
catalog-aware card; the /ai route builds adapters via buildAdapter()
and resolves API keys from the arcadia vault per-call (direct mode).
Anthropic skips the /v1/models probe (no such endpoint) and uses
catalog defaults; failed probes for keyed providers fall back to the
catalog instead of dropping to mock.

AI conversation now persists across navigation and refresh via a new
crema.ai.live localStorage key (separate from the compact-snapshot
key). useChat hydrates from initialMessages on mount, saves on every
change, and "Clear conversation" wipes both state and storage.

Vite needs explicit resolve.alias for @crema/llm-ui and
@crema/llm-providers-ui — when a sibling lib imports another @crema/*,
tsconfigPaths can't resolve it (the importing file isn't in this
project's tsconfig scope).

Adds docs/LLM_PROXY_CONTRACT.md describing the
POST /api/v1/ai/llm/chat endpoint the backend needs for proxy mode
(keys never leave the server). Direct mode works against today's
arcadia; proxy mode unblocks once that endpoint ships.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

app/app.css

@@ -18,6 +18,7 @@
 @source "../../lib-feedback-ui/src";
 @source "../../lib-auth-ui/src";
 @source "../../lib-agent-ui/src";
+@source "../../lib-llm-providers-ui/src";
 /* CREMA:SOURCES */
 
 @custom-variant dark (&:is(.dark *));