Various bug fixes found while testing SaaS build (#6459)

# Description of Changes
Various fixes and improvements I made while testing the SaaS code:
- Changes the new `.env.saas` file to live in `app/` and match the
semantics of the other `.env` files
- Adds top-level `task dev:saas` command to spawn SaaS frontend &
backend
- Deletes dead SaaS code and improves some overriding logic
- Fixes refreshing issue when coming back to the tab
- Fix the Compare tool's selection logic
- Make Compare handle error cases properly
- Fixes the location of the "Dismiss All Errors" button (was rendering
on top of the top-bar with a transparent background previously so it
looked rubbish)
- Fixes file selection in PDF Editor

frontend/editor/src/saas/routes/login/LoadingState.tsx

@@ -1,22 +0,0 @@
-import { useTranslation } from "@app/hooks/useTranslation";
-
-export default function LoadingState() {
-  const { t } = useTranslation();
-
-  return (
-    <div
-      style={{
-        minHeight: "100vh",
-        display: "flex",
-        alignItems: "center",
-        justifyContent: "center",
-        backgroundColor: "#f3f4f6",
-      }}
-    >
-      <div style={{ textAlign: "center" }}>
-        <div style={{ fontSize: "32px", marginBottom: "16px" }}>⏳</div>
-        <p style={{ color: "#6b7280" }}>{t("loading")}</p>
-      </div>
-    </div>
-  );
-}