feat(AppShell): add contentPadding prop

[cixzhang]

Summary

Adds a contentPadding prop to XDSAppShell that controls the main content area padding.

Motivation

Ran an ad-hoc vibe test comparing three padding mental models (10 prompts × 3 approaches = 30 tests). contentPadding won on mixed layouts — the hard case — with the fewest total decisions (15 vs 16 for no-default-padding vs 19 for default-padding).

Key insight: the LLM correctly picks contentPadding={4} for mostly-padded pages and contentPadding={0} for mostly-bleed pages, then overrides only the exceptions with XDSSection.

API

// Padded settings page
<XDSAppShell contentPadding={4} topNav={...}>
  <XDSHeading level={1}>Settings</XDSHeading>
  <XDSTextInput label="Name" />
</XDSAppShell>

// Mostly padded with a full-bleed chart
<XDSAppShell contentPadding={4} topNav={...}>
  <XDSHeading level={1}>Dashboard</XDSHeading>
  <XDSSection padding={0}>
    <FullWidthChart />
  </XDSSection>
  <XDSCard>Details</XDSCard>
</XDSAppShell>

// Mostly full-bleed with padded details
<XDSAppShell contentPadding={0} topNav={...}>
  <FullWidthVideoPlayer />
  <XDSSection padding={4}>
    <XDSHeading level={1}>Video Title</XDSHeading>
  </XDSSection>
</XDSAppShell>

Changes

• Add contentPadding?: SpacingStep to XDSAppShellProps
• Pass through to XDSLayoutContent padding (defaults to 0 for backward compat)
• Add 3 doc examples showing both directions (padded default + full-bleed override, and vice versa)
• Add 3 tests for contentPadding behavior
• Update English + Chinese docs

Test plan

• yarn build ✅
• yarn test — 90 files, 1436 tests passing ✅

Ref #565

[github-actions]

PR Analysis Report

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Modified Components

AppShell · View in Storybook

Metric	Before	After	Delta
Bundle Size (ESM)	N/A	625B	N/A
Lines of Code	N/A	386	-
Complexity	N/A	Very High (48)	-

Bundle Size Summary

Package	Size (ESM)	Size (CJS)	Gzipped
@xds/core	9.3KB	15.0KB	2.4KB

Accessibility Audit

Status: No accessibility violations detected.

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