Agents: CRUD, DB table, session binding, replace model picker

- agents table with name, avatar, description, model, persona, instructions, soft-delete
- AgentEndpoint (GET/POST/PUT/DELETE /api/agents)
- sessions + chains now reference agent_id
- ChatEndpoint resolves agent from session (no agent = no chat)
- system prompt built from agent persona + instructions
- frontend: agent editor with avatar upload, agent cards on dashboard
- session: agent dropdown replaces model picker, choice persisted

wwwroot/index.html

@@ -52,10 +52,7 @@
     <header class="session-header">
       <button class="back-btn" data-page="dashboard">←</button>
       <h1 id="sessionTitle" class="editable-title">Session</h1>
-      <select id="model">
-        <option value="sonnet">Sonnet 4.6</option>
-        <option value="opus">Opus 4.6</option>
-      </select>
+      <select id="agentSelect"></select>
       <button class="header-btn" id="archiveBtn">Archivieren</button>
       <div class="toggle">
         <input type="checkbox" id="showSystem">
@@ -76,12 +73,33 @@
   <div class="page agent-page">
     <header class="session-header">
       <button class="back-btn" data-page="dashboard">←</button>
-      <h1>Agent</h1>
+      <h1 id="agentPageTitle">Neuer Agent</h1>
     </header>
-    <div class="placeholder-page">
-      <div class="placeholder-icon">🤖</div>
-      <h2>Agents</h2>
-      <p>Hier wird der ausgewählte Agent angezeigt.</p>
+    <div class="agent-form">
+      <div class="agent-avatar-section">
+        <div class="agent-avatar-preview" id="avatarPreview">?</div>
+        <input type="file" id="avatarInput" accept="image/*" hidden>
+        <button id="avatarBtn">Avatar ändern</button>
+        <button id="avatarClearBtn">Entfernen</button>
+      </div>
+      <label>Name</label>
+      <input type="text" id="agentName" placeholder="Name des Agents">
+      <label>Beschreibung</label>
+      <input type="text" id="agentDescription" placeholder="Kurze Beschreibung (optional)">
+      <label>Model</label>
+      <select id="agentModel">
+        <option value="sonnet">Sonnet 4.6</option>
+        <option value="opus">Opus 4.6</option>
+      </select>
+      <label>Persona</label>
+      <textarea id="agentPersona" rows="6" placeholder="Persönlichkeit / Seele des Agents"></textarea>
+      <label>Instructions</label>
+      <textarea id="agentInstructions" rows="6" placeholder="Technische Anweisungen"></textarea>
+      <div class="agent-form-actions">
+        <button id="agentSaveBtn">Speichern</button>
+        <button id="agentCancelBtn">Abbrechen</button>
+        <button id="agentDeleteBtn" class="danger-btn" hidden>Löschen</button>
+      </div>
     </div>
   </div>
 </template>
@@ -107,6 +125,7 @@
 <script src="js/history.js"></script>
 <script src="js/router.js"></script>
 <script src="js/pages/dashboard.js"></script>
+<script src="js/pages/agent-page.js"></script>
 <script src="js/pages/session-page.js"></script>
 <script src="js/app.js"></script>
 </body>