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/public/locales/en-GB/translation.toml

@@ -2794,6 +2794,7 @@ selectBaseFirst = "Select original PDF first"
 filesMissing = "Unable to locate the selected files. Please re-select them."
 generic = "Unable to compare these files."
 selectRequired = "Select a original and edited document."
+title = "Comparison failed"
 
 [compare.large.file]
 message = "One or Both of the provided documents are too large to process"
@@ -2859,6 +2860,7 @@ title = "Still working…"
 
 [compare.status]
 complete = "Comparison ready"
+error = "Comparison failed"
 extracting = "Extracting text..."
 processing = "Analysing differences..."