Check component props for untranslated literals, and fail on a finding

i18n-literals checked title, aria-label, placeholder and alt on elements,
but not the same English passed to a component, so a MenuItem label or a
Popover ariaLabel written as a literal went through. It also excused a
literal that happened to be a catalogue key. That exemption is meant for
English held in a constant and translated where it renders, and a literal
written straight into a JSX attribute has no such render site. No
component passes its props through t(). And neither half of i18n:check
was run with --check, so a finding printed and the script still exited 0.

Component props are checked now, a key no longer excuses a literal in an
attribute, and both scripts run with --check. That found 28 strings
rendering English in every language: 19 already had keys and are wrapped,
and 9 are new keys in all nine catalogues. The contact editor's Save and
Saving… buttons are wrapped as well, on the same line.

CONTRIBUTING.md

@@ -92,7 +92,7 @@ in all nine languages. Nothing errors. The only signal is the fallback count
 going up, so read it:
 
 ```sh
-npm run i18n:check                    # literals wrapped, and catalogue health
+npm run i18n:check                    # literals wrapped, and catalogue health; exits 1 on a finding
 node scripts/i18n-catalog-check.mjs   # per-language: translated / used / falling back
 ```