Per-session workspace, tools only when workspace set

- Session has workspace field (absolute path, optional)
- ToolService stateless, receives workspace per call
- No workspace = no tools sent to API
- Removed hardcoded workspace from Program.cs
- Workspace field in session edit form

wwwroot/js/pages/session-page.js

@@ -16,6 +16,7 @@ registerPage('session', {
     const titleEl = document.getElementById('sessionPageTitle');
     const nameEl = document.getElementById('sessionName');
     const descEl = document.getElementById('sessionDescription');
+    const workspaceEl = document.getElementById('sessionWorkspace');
     const agentSelect = document.getElementById('agentSelect');
     const imagePreview = document.getElementById('sessionImagePreview');
     const imageInput = document.getElementById('sessionImageInput');
@@ -32,6 +33,7 @@ registerPage('session', {
       titleEl.textContent = session.name || 'Unbenannt';
       nameEl.value = session.name || '';
       descEl.value = session.description || '';
+      workspaceEl.value = session.workspace || '';
       _sessionImageData = session.image || null;
       deleteBtn.hidden = false;
       openChatBtn.hidden = false;
@@ -81,7 +83,8 @@ registerPage('session', {
         name: nameEl.value.trim() || 'Unbenannt',
         description: descEl.value.trim(),
         image: _sessionImageData,
-        agentId: agentSelect.value || null
+        agentId: agentSelect.value || null,
+        workspace: workspaceEl.value.trim() || null
       };
 
       if (isNew) {