fix: require valid project directory for session creation

- Backend: POST /api/chat/sessions now validates working_directory
  exists (400 MISSING_DIRECTORY / INVALID_DIRECTORY)
- db.ts: remove process.cwd() fallback in createSession
- ChatListPanel: validate last-working-directory before use,
  open FolderPicker when invalid; add SSR guard for localStorage
- NewChatPage: block sending without a project directory selected

Bump version to 0.10.5.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/app/api/chat/sessions/route.ts

@@ -1,4 +1,5 @@
 import { NextRequest } from 'next/server';
+import fs from 'fs/promises';
 import { getAllSessions, createSession } from '@/lib/db';
 import type { CreateSessionRequest, SessionsResponse, SessionResponse } from '@/types';
 
@@ -17,6 +18,25 @@ export async function GET() {
 export async function POST(request: NextRequest) {
   try {
     const body: CreateSessionRequest = await request.json();
+
+    // Validate working_directory is provided
+    if (!body.working_directory) {
+      return Response.json(
+        { error: 'Working directory is required', code: 'MISSING_DIRECTORY' },
+        { status: 400 },
+      );
+    }
+
+    // Validate directory actually exists on disk
+    try {
+      await fs.access(body.working_directory);
+    } catch {
+      return Response.json(
+        { error: 'Directory does not exist', code: 'INVALID_DIRECTORY' },
+        { status: 400 },
+      );
+    }
+
     const session = createSession(
       body.title,
       body.model,