fix: hide encrypted post summaries

[sleep1223]

Type of change

• Bug fix (a non-breaking change that fixes an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality not to work as expected)
• Other (please describe):

Checklist

• I have read the CONTRIBUTING document.
• I have checked to ensure that this Pull Request is not for personal changes.
• I have performed a self-review of my own code.
• My changes generate no new warnings.

Related Issue

N/A

Changes

• Add hideHomeContent frontmatter support for encrypted posts.
• Default encrypted posts with a configured password to hide public summary content.
• Replace public summary output with 该文章已加密 across homepage cards, post lists, detail metadata, feeds, API metadata, OG images, and archive island props.
• Avoid passing raw post descriptions/password-related data into the archive Svelte island.
• Add regression tests for default and explicit hideHomeContent behavior.
• Update the encrypted post example with the new top-level config.

How To Test

• node --test --experimental-strip-types tests/post-card-content.test.ts
• pnpm check
• pnpm build
• Ran targeted ESLint on changed files; it completed with only existing console warnings in feed/build scripts.
• Verified built output no longer contains the raw encrypted example summary text.

Screenshots (if applicable)

N/A

Additional Notes

The placeholder text is currently fixed as 该文章已加密, matching the requested display text. A future follow-up can route it through the existing i18n system if maintainers want localized placeholder text.