web: align captcha stage with post-21380 main drift

Picks up the non-PR-#21380 changes to captcha files that are already on
main: `.style-scope` selector variants in CaptchaStage.css (from #20134)
and `export default CaptchaStage` (from #20397). Both are functionally
inert on this branch — no code applies the style-scope class to
ak-stage-captcha, and no importer uses the default export — but
including them keeps the cherry-pick zero-drift against main.

Author: GirlBossRush

web/src/flow/stages/captcha/CaptchaStage.css

@@ -1,9 +1,11 @@
-:host {
+:host,
+ak-stage-captcha.style-scope {
     --captcha-background-to: var(--pf-global--BackgroundColor--light-100);
     --captcha-background-from: var(--pf-global--BackgroundColor--light-300);
 }
 
-:host([theme="dark"]) {
+:host([theme="dark"]),
+ak-stage-captcha[theme="dark"].style-scope {
     --captcha-background-to: var(--ak-dark-background-light);
     --captcha-background-from: var(--pf-global--BackgroundColor--300);
 }

web/src/flow/stages/captcha/CaptchaStage.ts

@@ -560,6 +560,8 @@ export class CaptchaStage
     }
 }
 
+export default CaptchaStage;
+
 declare global {
     interface HTMLElementTagNameMap {
         "ak-stage-captcha": CaptchaStage;