Merge refactor/deepen-architecture: deepen settings, session, providers, search

Three-phase architecture refactor (net -245 lines), build + lint green: Phase 1 unified the settings schema behind one module with per-request resolveSettings; Phase 2 added the prepareSession module and a single prompt builder, fixing the mid-session stale-prompt bug; Phase 3 added a shared provider streaming core and a SearchProvider seam, deleting the dead TogetherAIStream.ts. Also fixes a pre-existing react-hooks/exhaustive-deps warning in Chat.tsx.

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

Author: Michael Borck

app/api/getChat/route.ts

@@ -2,41 +2,21 @@ import {
   createProvider,
   LLMStreamPayload,
 } from "@/utils/providers";
-import { getSettings } from "@/utils/settings";
+import { resolveSettings } from "@/utils/settings";
 
 export async function POST(request: Request) {
   let { messages } = await request.json();
 
   try {
-    // First, try to get settings from request headers (if frontend sends them)
-    const settingsHeader = request.headers.get('X-StudyBuddy-Settings');
-    if (settingsHeader) {
-      try {
-        const frontendSettings = JSON.parse(settingsHeader);
-        const { updateSettings } = await import("@/utils/settings");
-        updateSettings(frontendSettings);
-        console.log('Applied frontend settings:', frontendSettings.llmProvider);
-      } catch (e) {
-        console.warn('Failed to parse frontend settings:', e);
-      }
-    }
-    
-    const settings = getSettings();
-    console.log('Using settings:', { 
-      provider: settings.llmProvider, 
-      baseUrl: settings.llmBaseUrl, 
-      model: settings.llmModel,
-      hasApiKey: !!settings.llmApiKey 
-    });
-    
-    const provider = createProvider(); // Will use runtime settings
-    
+    const settings = resolveSettings(request);
+    const provider = createProvider(settings);
+
     const payload: LLMStreamPayload = {
       model: settings.llmModel,
       messages,
       stream: true,
     };
-    
+
     const stream = await provider.stream(payload);
 
     return new Response(stream, {