feat: add nested navigation and Command Palette

[Vonng]

Summary

Implements PRD 4 using Hugo-native information architecture:

• add one-level desktop navbar disclosures and matching mobile accordions
• preserve flat menu compatibility and degrade deeper nesting to static groups with warnings
• add configurable sidebar icon density
• extend the offline multilingual search schema with roots, breadcrumbs, keywords, boosts, aliases, and grouped ranking
• unify PRD 4 page actions behind a safe action registry
• upgrade the existing local-search dialog into a Command Palette with empty, text, command, and choice modes
• publish matching EN/ZH migration documentation and an explicit starter configuration
• freeze behavior in machine-checkable fixtures and runtime gates

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Compatibility and safety

• Hugo Menu remains the global navigation authority; content/root navigation remains Hugo-native.
• Existing flat menus and legacy search exclusion aliases remain supported.
• Search stays offline, language-separated, CJK-capable, and telemetry-free.
• Site commands accept only safe URL records or allowlisted built-in action IDs; configuration cannot inject callbacks.
• Palette/index runtime is omitted when local search is disabled and from print output.

Validation

Local CI-equivalent suites passed with Hugo 0.160.1+extended and 0.164.0+extended, including:

• PRD 4 contract, runtime, sidebar, search, action, palette, docs, and i18n checks
• warning-free example builds
• existing code-block, content primitive, media, image zoom, gallery, font-token, and typography checks
• consumer integration under root and /preview/, EN/ZH, and 81 Playwright checks per Hugo version (29 axe/WCAG + 52 functional)

The consumer-site integration remains in a separate dependent branch and will be updated to this merged theme commit before its own PR is merged.