Delete dead translations (#6581)

# Description of Changes
Adds all the missing translations that I could find (they're all dynamic
ones that the existing test can't detect are required) and adds a new
test to find as many unused translations as possible. The test has an
ignore list for translations that are used, but dynamically so the test
can't find them (most of the settings UI translations are built up
dynamically like that).

This PR is scoped to just include en-GB translation changes, since
that's the main supported language. We'll need to do a translation PR to
trim all the dead keys from the other languages, and add the missing
ones.

.taskfiles/frontend.yml

@@ -327,19 +327,19 @@ tasks:
 
   test:
     desc: "Run tests"
-    deps: [install]
+    deps: [prepare]
     cmds:
       - npx vitest run --root editor
 
   test:watch:
     desc: "Run tests in watch mode"
-    deps: [install]
+    deps: [prepare]
     cmds:
       - npx vitest --watch --root editor
 
   test:coverage:
     desc: "Run tests with coverage (one-shot; CI-friendly)."
-    deps: [install]
+    deps: [prepare]
     cmds:
       # `vitest run` makes this CI-safe (the bare `vitest` form enters watch
       # mode). Explicit reporter list because v8 + json-summary is what the