refactor(web): resolve Mantine/CSS rule violations in mid-session auth UI

- App.tsx: replace the inline `style={{}}` on the re-auth banner wrapper with
  Mantine style props (pos/top/bg) plus a `.reauth-banner-bar` class for the
  z-index/box-shadow that have no Mantine shorthand.
- App.tsx: extract the ~100-line inline `onAuthorize` and the inline `onCancel`
  step-up handlers into named `handleStepUpAuthorize`/`handleStepUpCancel`.
- ReAuthBanner: move the flat `styles={{ root }}` (background/border) into a
  `reauth` Alert theme variant; replace the raw `<span>` with `<Text>`.
- StepUpAuthModal: extract the inline `onClick` wrapper into `handleAuthorize`.

No behavior change; validate + integration green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01DZyeWManYTvcXxjcrhxSQc

clients/web/src/App.css

@@ -412,3 +412,13 @@
 .server-drag-handle:active {
   cursor: grabbing;
 }
+
+/*
+ * Sticky re-auth banner bar. Positioning (pos/top) and background are set via
+ * Mantine props on the Box; z-index and box-shadow have no Mantine style-prop
+ * shorthand, so they live here.
+ */
+.reauth-banner-bar {
+  z-index: 200;
+  box-shadow: var(--mantine-shadow-sm);
+}