Add LLM provider switch, markdown chat UI, and update README

- Dual RAG instances (Ollama + OpenAI) for on-the-fly provider switching
- Provider dropdown in chat UI, /api/providers endpoint
- Markdown rendering for assistant responses
- Server logs include provider and model name for each LLM response
- README: OpenAI setup, add_pdfs, API docs, provider switch

Co-authored-by: Cursor <cursoragent@cursor.com>

templates/chat.html

@@ -112,6 +112,24 @@
       border-top: 1px solid #27272a;
       background: #18181b;
     }
+    #provider-row {
+      display: flex;
+      align-items: center;
+      gap: 0.5rem;
+      margin-bottom: 0.5rem;
+      font-size: 0.8rem;
+      color: #71717a;
+    }
+    #provider-row label { flex-shrink: 0; }
+    #provider {
+      padding: 0.25rem 0.5rem;
+      font: inherit;
+      font-size: 0.85rem;
+      color: #e4e4e7;
+      background: #27272a;
+      border: 1px solid #3f3f46;
+      border-radius: 6px;
+    }
     #input-row {
       display: flex;
       gap: 0.5rem;
@@ -168,6 +186,13 @@
   <div id="messages"></div>
 
   <div id="input-area">
+    <div id="provider-row">
+      <label for="provider">LLM provider:</label>
+      <select id="provider">
+        <option value="ollama">Ollama</option>
+        <option value="openai">OpenAI</option>
+      </select>
+    </div>
     <div id="input-row">
       <textarea id="input" rows="1" placeholder="Ask a question…" autofocus></textarea>
       <button type="button" id="send">Send</button>
@@ -178,8 +203,22 @@
     const messagesEl = document.getElementById('messages');
     const inputEl = document.getElementById('input');
     const sendBtn = document.getElementById('send');
+    const providerEl = document.getElementById('provider');
     const chatHistory = [];
 
+    (async function initProviders() {
+      try {
+        const res = await fetch('/api/providers');
+        const data = await res.json();
+        if (!data.openai) {
+          const opt = providerEl.querySelector('option[value="openai"]');
+          opt.disabled = true;
+          opt.textContent = 'OpenAI (not configured)';
+          if (providerEl.value === 'openai') providerEl.value = 'ollama';
+        }
+      } catch (_) {}
+    })();
+
     function appendMessage(role, text, isError = false) {
       text = text ?? '';
       const div = document.createElement('div');
@@ -231,7 +270,11 @@
         const res = await fetch('/api/chat', {
           method: 'POST',
           headers: { 'Content-Type': 'application/json' },
-          body: JSON.stringify({ message: text, history: history })
+          body: JSON.stringify({
+            message: text,
+            history: history,
+            llm_provider: providerEl.value
+          })
         });
         const data = await res.json();
         setLoading(false);