fix: decouple project name from folder path in project creation

Remove automatic subfolder creation when creating projects. Users now
select the exact folder they want to use, enabling support for existing
projects without requiring folder names to match project names.

Changes:
- NewProjectModal.tsx: Remove path concatenation that appended project
  name to selected folder. Update instruction text to clarify users
  select THE project folder, not a parent location.
- FolderBrowser.tsx: Add visual indicator "This folder will contain
  all project files" to clarify selection behavior.
- projects.py: Add duplicate path validation to prevent multiple
  projects from registering the same directory. Includes case-insensitive
  path comparison on Windows for proper cross-platform support.

This allows users to:
- Use Auto Coder on existing projects by selecting their folder directly
- Have project names that differ from folder names (name is a registry label)
- Get clear feedback when a path is already registered under another name

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

server/routers/projects.py

@@ -8,6 +8,7 @@ Uses project registry for path lookups instead of fixed generations/ directory.
 
 import re
 import shutil
+import sys
 from pathlib import Path
 
 from fastapi import APIRouter, HTTPException
@@ -131,7 +132,7 @@ async def list_projects():
 async def create_project(project: ProjectCreate):
     """Create a new project at the specified path."""
     _init_imports()
-    register_project, _, get_project_path, _, _ = _get_registry_functions()
+    register_project, _, get_project_path, list_registered_projects, _ = _get_registry_functions()
 
     name = validate_project_name(project.name)
     project_path = Path(project.path).resolve()
@@ -144,6 +145,22 @@ async def create_project(project: ProjectCreate):
             detail=f"Project '{name}' already exists at {existing}"
         )
 
+    # Check if path already registered under a different name
+    all_projects = list_registered_projects()
+    for existing_name, info in all_projects.items():
+        existing_path = Path(info["path"]).resolve()
+        # Case-insensitive comparison on Windows
+        if sys.platform == "win32":
+            paths_match = str(existing_path).lower() == str(project_path).lower()
+        else:
+            paths_match = existing_path == project_path
+
+        if paths_match:
+            raise HTTPException(
+                status_code=409,
+                detail=f"Path '{project_path}' is already registered as project '{existing_name}'"
+            )
+
     # Security: Check if path is in a blocked location
     from .filesystem import is_path_blocked
     if is_path_blocked(project_path):