P0: Add StaticFiles mount + root redirect to install wizard

Critical fix: FastAPI was not serving any static files, causing all
frontend pages (install.html, login.html, etc.) to return 404.

Changes:
- Add StaticFiles mount at /app/ for web/app/ directory (html=True)
- Redirect root path / to /app/install.html in install mode
- Remove redundant "/" path bypass in InstallModeMiddleware
- Production Nginx still serves static files directly (no conflict)

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

server/main.py

@@ -25,6 +25,7 @@ from pathlib import Path
 
 from fastapi import FastAPI, Request, Response
 from fastapi.middleware.cors import CORSMiddleware
+from fastapi.staticfiles import StaticFiles
 from starlette.middleware.base import BaseHTTPMiddleware
 
 from server.config import settings
@@ -268,10 +269,14 @@ class InstallModeMiddleware(BaseHTTPMiddleware):
     async def dispatch(self, request: Request, call_next):
         if INSTALL_MODE:
             path = request.url.path
+            # Redirect root to install wizard
+            if path == "/":
+                from fastapi.responses import RedirectResponse
+                return RedirectResponse(url="/app/install.html")
             # Allow: install API, static files, health check
             if path.startswith("/api/install/") or path.startswith("/app/") or path == "/health":
                 return await call_next(request)
-            if path == "/" or path.startswith("/ws/"):
+            if path.startswith("/ws/"):
                 return await call_next(request)
             # Block everything else
             from fastapi.responses import JSONResponse
@@ -327,4 +332,10 @@ app.include_router(webssh_router)
 app.include_router(sync_v2_router)
 
 # Global Search
-app.include_router(search_router)
+app.include_router(search_router)
+
+# ── Static files: serve web/app/ at /app/ (HTML pages + JS/CSS assets) ──
+# In production, Nginx serves these directly; this is for dev/standalone mode.
+WEB_APP_DIR = ROOT_DIR / "web" / "app"
+if WEB_APP_DIR.is_dir():
+    app.mount("/app", StaticFiles(directory=str(WEB_APP_DIR), html=True), name="static_app")