docs: localize first-app Quickstart screenshots

[pbc1017]

Summary

• Replace stale shared setup screenshots with current Korean, English, and Japanese Developer Portal captures.
• Point each localized Quickstart guide at the matching locale assets.
• Keep tutorial WAM and result captures shared because that tutorial UI is currently locale-neutral.

Privacy and safety

• Other internal app names are blurred.
• App ID and endpoints use non-production example values.
• App Secret is masked.
• No Developer Portal settings were submitted while capturing.

Test plan

• make docs-check
• git diff --check
• Visually inspect all 18 localized setup screenshots
• Restore the browser account language to Korean

[channeltalk]

https://desk.channel.io/#/channels/1/team_chats/groups/371781/6a8befa3e6c79dd1a301

[coderabbitai]

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📥 Commits

Reviewing files that changed from the base of the PR and between 725ca5a and 877b4a7.

⛔ Files ignored due to path filters (18)

• docs/assets/first-app/en/app-id.png is excluded by !**/*.png
• docs/assets/first-app/en/app-secret.png is excluded by !**/*.png
• docs/assets/first-app/en/app-store-entry.png is excluded by !**/*.png
• docs/assets/first-app/en/create-app.png is excluded by !**/*.png
• docs/assets/first-app/en/endpoints.png is excluded by !**/*.png
• docs/assets/first-app/en/permissions.png is excluded by !**/*.png
• docs/assets/first-app/ja/app-id.png is excluded by !**/*.png
• docs/assets/first-app/ja/app-secret.png is excluded by !**/*.png
• docs/assets/first-app/ja/app-store-entry.png is excluded by !**/*.png
• docs/assets/first-app/ja/create-app.png is excluded by !**/*.png
• docs/assets/first-app/ja/endpoints.png is excluded by !**/*.png
• docs/assets/first-app/ja/permissions.png is excluded by !**/*.png
• docs/assets/first-app/ko/app-id.png is excluded by !**/*.png
• docs/assets/first-app/ko/app-secret.png is excluded by !**/*.png
• docs/assets/first-app/ko/app-store-entry.png is excluded by !**/*.png
• docs/assets/first-app/ko/create-app.png is excluded by !**/*.png
• docs/assets/first-app/ko/endpoints.png is excluded by !**/*.png
• docs/assets/first-app/ko/permissions.png is excluded by !**/*.png

📒 Files selected for processing (3)

• docs/guides/en/quickstart.md
• docs/guides/ja/quickstart.md
• docs/guides/ko/quickstart.md

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