LLM: keep API key in localStorage only, default to muse-spark-1.3

Secrets must never be baked into the static build. Stop reading
VITE_AI_API_KEY from env — key comes only from localStorage via the
⚙️ config dialog (flow-state:llm). Gateway URL / model may still come
from env as non-secret defaults.

- src/llm/config.js: DEFAULTS model → meta/muse-spark-1.3-contributor
  (cheap, per user request), readEnv no longer touches apiKey,
  getLlmConfig resolves key only from stored.apiKey.
- .env.example: drop VITE_AI_API_KEY=, document browser-only key flow.
- src/main.js: dialog copy now says key is browser-only, never baked.

Build + lint:scenes + tests still green. No VITE_AI_API_KEY string
remains in source.

Co-Authored-By: Claude <noreply@anthropic.com>

flow-state/src/llm/config.js

@@ -1,14 +1,16 @@
-// LLM gateway config — localStorage overrides > VITE env > defaults.
+// LLM gateway config — localStorage for the secret, env only for non-secrets.
 //
-// The app is a static Vite site with no server; the gateway credential lives
-// in the browser. Never commit a key — paste it in the panel (→ localStorage)
-// or in an untracked .env.local.
+// The app is a static Vite site with no server. The API key is a secret and
+// must never be baked into the build — it lives only in the browser's
+// localStorage (→ ⚙️ dialog). Gateway URL and model are non-secret and may
+// come from VITE env as defaults with a localStorage override; the key never
+// does.
 
 const STORAGE_KEY = 'flow-state:llm';
 
 const DEFAULTS = {
     gatewayUrl: 'https://ai-gateway.vercel.sh/v1',
-    model: 'openai/gpt-4o-mini',
+    model: 'meta/muse-spark-1.3-contributor',
     apiKey: '',
 };
 
@@ -18,10 +20,10 @@ function readEnv() {
         return {
             gatewayUrl: env.VITE_AI_GATEWAY_URL || null,
             model: env.VITE_AI_MODEL || null,
-            apiKey: env.VITE_AI_API_KEY || null,
+            // apiKey intentionally NOT read from env — see module comment.
         };
     } catch {
-        return { gatewayUrl: null, model: null, apiKey: null };
+        return { gatewayUrl: null, model: null };
     }
 }
 
@@ -36,14 +38,13 @@ function readStorage() {
     }
 }
 
-/** Resolved config — localStorage wins, then env, then defaults. */
+/** Resolved config — key from localStorage only (never env); URL/model from localStorage > env > defaults. */
 export function getLlmConfig() {
     const env = readEnv();
     const stored = readStorage();
     const gatewayUrl = stored.gatewayUrl || env.gatewayUrl || DEFAULTS.gatewayUrl;
     const model = stored.model || env.model || DEFAULTS.model;
-    const apiKey = stored.apiKey != null ? stored.apiKey : (env.apiKey != null ? env.apiKey : DEFAULTS.apiKey);
-    const key = typeof apiKey === 'string' ? apiKey.trim() : '';
+    const key = typeof stored.apiKey === 'string' ? stored.apiKey.trim() : '';
     return {
         gatewayUrl: String(gatewayUrl || DEFAULTS.gatewayUrl).trim() || DEFAULTS.gatewayUrl,
         model: String(model || DEFAULTS.model).trim() || DEFAULTS.model,