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

.taskfiles/backend.yml

@@ -40,10 +40,10 @@ tasks:
         platforms: [linux, darwin]
 
   dev:saas:
-    desc: "Start backend in SaaS flavor against Supabase (loads .env.saas.local)"
+    desc: "Start backend in SaaS flavor against Supabase"
     # `dotenv:` reads from the root Taskfile's directory (".") because this
-    # subtaskfile is included with `dir: .`. Drop the file at the repo root.
-    dotenv: ['.env.saas.local']
+    # subtaskfile is included with `dir: .`.
+    dotenv: ['app/.env.saas.local', 'app/.env.saas']
     ignore_error: true
     vars:
       PORT: '{{.PORT | default "8080"}}'