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

Taskfile.yml

@@ -58,6 +58,23 @@ tasks:
           BACKEND_URL: 'http://localhost:{{.BACKEND_PORT}}'
           OPEN: "true"
 
+  dev:saas:
+    desc: "Start SaaS backend + frontend concurrently on free ports"
+    vars:
+      PORTS:
+        sh: '{{if eq OS "windows"}}{{.FIND_FREE_PORT_PS}} 8080 5173{{else}}{{.FIND_FREE_PORT_SH}} 8080 5173{{end}}'
+      BACKEND_PORT: '{{index (splitList "\n" .PORTS) 0}}'
+      FRONTEND_PORT: '{{index (splitList "\n" .PORTS) 1}}'
+    deps:
+      - task: backend:dev:saas
+        vars:
+          PORT: '{{.BACKEND_PORT}}'
+      - task: frontend:dev:saas
+        vars:
+          PORT: '{{.FRONTEND_PORT}}'
+          BACKEND_URL: 'http://localhost:{{.BACKEND_PORT}}'
+          OPEN: "true"
+
   dev:all:
     desc: "Start backend + frontend + engine concurrently on free ports"
     vars: