feat: 404 pages → redirect to /workspaces

- main.py: @app.exception_handler(404) → RedirectResponse(/workspaces)
- dashboard.py: page_detail 404 → RedirectResponse(/workspaces)
- tests: test_styled_404_page → 302 redirect
- API paths (containing /api) still get JSON 404, not redirect
- 143 tests passent

app/main.py

@@ -152,10 +152,10 @@ body{font-family:-apple-system,BlinkMacSystemFont,'Segoe UI',sans-serif;backgrou
 
 @app.exception_handler(404)
 async def not_found_handler(request: Request, exc):
-    """Styled 404 page matching the FlowDeck dark theme."""
-    from fastapi.responses import HTMLResponse
-    # API routes should get JSON, not HTML
-    if request.url.path.startswith("/api/") or request.url.path.startswith("/board/api/"):
+    """Redirect 404 HTML pages to /workspaces. API routes still get JSON."""
+    # Preserve JSON 404 for all API-like paths (including /db/xxx/api)
+    if "/api" in request.url.path:
         from fastapi.responses import JSONResponse
         return JSONResponse({"detail": "Not found"}, status_code=404)
-    return HTMLResponse(NOT_FOUND_HTML, status_code=404)
+    from fastapi.responses import RedirectResponse
+    return RedirectResponse("/workspaces", status_code=302)