fix(browser): match the unknown decrypt reason explicitly

CI's type-aware code-quality gate flagged the reason switch as non-exhaustive:
the 'unknown' member was handled by `default:` rather than matched.

Matching it explicitly keeps the behaviour identical today and makes the gate
enforce the thing that matters — adding a new reason to the union now fails the
switch instead of falling silently into a generic message that would not
describe it.

This gate is separate from `oxlint` and is not covered by running oxlint on the
changed files, which is why it only surfaced in CI.

src/renderer/src/lib/browser-cookie-import-toast.ts

@@ -47,7 +47,7 @@ function formatCookieImportWarning(warning: CookieImportWarning): string {
                 '{{value0}} cookies could not be decrypted because the system keyring was unavailable. Unlock your login keyring (or install a Secret Service provider such as gnome-keyring) and import again.',
                 { value0: warning.failedCookies }
               )
-        default:
+        case 'unknown':
           return translate(
             'auto.lib.browser.cookie.import.toast.undecryptableUnknown',
             '{{value0}} cookies could not be decrypted and were skipped. Close the source browser completely and try the import again.',