fix(dashboard): scope native scrollbar styles to Firefox - #10760

Fixes #10758

On Safari (Safari >= 18.2), the inner scrollbar of the config page is
clipped by the rounded settings cards: it is visible above the cards and
disappears once it overlaps them. Chromium renders the same scrollbar above the
cards. This affects /config and other pages that combine an internal scroll
container with rounded cards (e.g. /settings).

Affected: AstrBot >= v3.5.16 (global scrollbar-width rule) on Safari >= 18.2
with overlay scrollbars; verified on macOS 27.0.1 + Safari 27.0.1.

Root cause:

• dashboard/src/scss/components/_VScrollbar.scss applies
  scrollbar-width: thin / scrollbar-color to *. Per CSS Scrollbars 1
  (§2.1/§3.1), non-auto values require browsers to ignore
  ::-webkit-scrollbar, so Chrome 121+/Safari 18.2+ fall back to the native
  overlay scrollbar instead of the intended 10px themed scrollbar.
• Overlay scrollbars have no gutter, so the rounded v-cards (Vuetify default
  position: relative; z-index: 0) extend underneath the scrollbar.
• Chromium composites overlay scrollbars above content; WebKit paints the
  scroller's scrollbar before positioned descendants, so Safari clips it at the
  card's rounded corner (see WebKit bug 297440, WONTFIX, and SO 60192029).

Modifications / 改动点

• dashboard/src/scss/components/_VScrollbar.scss: gate the standard scrollbar
  properties behind @supports not selector(::-webkit-scrollbar) so only
  Firefox uses them. WebKit/Blink go back to the existing themed
  ::-webkit-scrollbar styles (10px classic scrollbar with a gutter), which
  keeps content from covering the scrollbar and fixes the clipping.

• No JS, API, i18n or dependency changes. One file changed (+7/−4).

• Note: this restores the previously intended classic 10px themed scrollbar in
  Chrome/Safari instead of the native overlay scrollbar. It is a visual change
  (scrollbar appearance), not a functional breaking change.

• This is NOT a breaking change. / 这不是一个破坏性变更。

Screenshots or Test Results / 运行截图或测试结果

Verification steps:

1. Build the Dashboard: cd dashboard && pnpm install --frozen-lockfile && pnpm run build
2. Start the backend with the fresh build: uv run main.py --webui-dir dashboard/dist
3. Open /#/config in Safari and Chromium; scroll and observe the scrollbar next to the rounded cards.

Results:

• Safari: before → scrollbar clipped at the card's rounded corner;
  after → full 10px themed classic scrollbar visible, no clipping.
• Chromium: scrollbar still works; now renders as the 10px themed classic
  scrollbar (visual change noted above).
• Safari console: CSS.supports('selector(::-webkit-scrollbar)') === true,
  confirming the @supports branch is evaluated as intended.
• Firefox: keeps scrollbar-width: thin; scrollbar-color: ... by design;
  not manually tested.

Automated checks:

• cd dashboard && pnpm install --frozen-lockfile && pnpm run i18n:check && pnpm run build (passed)
• node --test dashboard/tests/*.test.mjs (83 passed)
• ruff format --check . / ruff check . (no Python files changed)

Screenshots:

Before (Safari only):

After (Safari/Chromium):

Checklist / 检查清单

• 😊 If there are new features added in the PR, I have discussed it with the authors through issues/emails, etc.
  / 如果 PR 中有新加入的功能，已经通过 Issue / 邮件等方式和作者讨论过。

• 👀 My changes have been well-tested, and "Verification Steps" and "Screenshots" have been provided above.
  / 我的更改经过了良好的测试，并已在上方提供了“验证步骤”和“运行截图”。

• 📚 I checked the affected WebUI instructions and screenshots in docs/zh and docs/en against the changed navigation, page structure, and labels, and updated them in this PR (or explained why no documentation update is needed). For renamed, moved, or merged entry points, I included an old entry → new entry mapping in the documentation and changelog.
  / 我已对照变化后的 WebUI 入口、页面结构和术语，核对并在本 PR 中更新 docs/zh 和 docs/en 的相关操作说明与截图（或说明无需更新文档的原因）。入口改名、移动或合并时，已在文档和 changelog 中补充 旧入口 → 新入口 对照。

• 🤓 I have ensured that no new dependencies are introduced, OR if new dependencies are introduced, they have been added to the appropriate locations in requirements.txt and pyproject.toml.
  / 我确保没有引入新依赖库，或者引入了新依赖库的同时将其添加到 requirements.txt 和 pyproject.toml 文件相应位置。

• 😮 My changes do not introduce malicious code.
  / 我的更改没有引入恶意代码。

Summary by Sourcery

Scope standard scrollbar properties to Firefox so Safari and Chromium use the existing themed scrollbar without clipping.

Bug Fixes:

• Prevent Safari scrollbar clipping around rounded dashboard cards by preserving the themed classic scrollbar in WebKit and Blink browsers.

Enhancements:

• Limit standard scrollbar styling to browsers without WebKit scrollbar support, while retaining Firefox-specific scrollbar customization.