refactor(auth): refresh auth entry layout

[quan0715]

變更目標

• 修正 auth entry 在 iPad/mobile 版型下圖片、內容區與 fixed action footer 的高度關係，避免底部內容被吃掉。
• 將註冊頁的校園認證改成通用入口，進入 /register/campus-sso 再選學校，避免直接展開 NYCU provider。
• 清理登入相關 i18n 舊 key，補上 login email divider 文案。
• 將原本集中在 AuthPages.scss 的大型 SCSS 拆成 layout/forms/providers/callback，並收斂重複 mobile screen 特例。

非目標

• 不處理 contest/backend 既有 worktree 變更。
• 不新增大量測例或擴大重構登入流程。
• 不處理 repo-wide 既有 naming lint debt。

影響範圍

• frontend/src/features/auth/**
• frontend/src/i18n/locales/*/common.json
• 登入、註冊、校園 SSO、OAuth authorize/callback 的 auth shell 視覺與行動版捲動行為。

驗證

• npm test -- AuthProviderOptionsScreen.test.tsx：通過，7 tests。
• npm run check:i18n：通過。
• bash .codex/skills/qjudge-quality-gates-owner/scripts/check-carbon-style.sh：通過。
• npm run build：通過；仍有既有 _scrollbar.scss slash-div deprecation 與 chunk size warning。
• mobile Playwright geometry check：通過，card bottom 與 footer top 對齊，confirm password 可捲到 footer 上方。
• pre-push hook：ESLint / TypeScript / frontend build / Docker Compose config 通過；hook 內 E2E 依既有設定跳過。
• node .codex/skills/qjudge-quality-gates-owner/scripts/lint-architecture.js --root frontend/src：通過。
• node .codex/skills/qjudge-quality-gates-owner/scripts/lint-naming.js --root frontend/src：失敗於 repo-wide 既有命名違規，未指向本次新增 auth SCSS 檔。

風險與回滾

• 主要風險在 auth shell responsive layout；已用 build、測試與 mobile geometry check 覆蓋。
• 回滾方式：revert 此 PR，可回到原 auth layout 與 i18n 結構。

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Filter registration SSO choices to supported providers

When auth options contain multiple campus providers and only some have supports_registration: true, this new registration button sends users to /register/campus-sso, but CampusSsoScreen still renders every provider with category === "campus"; since the OAuth callback creates or links users without checking supports_registration, login-only campus providers become selectable for account creation. The registration flow needs to pass/filter the supported providers on this screen before exposing them.

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