fix: decode html entities in post excerpts

[Pranjal1423]

Closes #430

Description

This PR fixes a visual bug where HTML entities in post excerpts (such as smart quotes “ / ” or apostrophes ’) were rendered raw inside the sub-row expanded view of the native Posts window.

Root Causes & Fixes

Missing Entity Decoding in Excerpt Render:

• Issue: The REST API returns post excerpts with HTML-encoded entities in the excerpt.rendered payload. While the Posts window stripped HTML tags before rendering, it did not decode the remaining HTML entities, causing them to render literally (e.g. “ showing on screen).
• Fix: Added a shared decodeHTML utility inside src/utils.ts and updated the buildSubRow function in src/posts-window/index.ts to decode the text before setting it as .textContent. Additionally, refactored decodeTitle in the same file to reuse the shared decodeHTML utility.

Verification

• Build: npm run build completed successfully.
• Typecheck: `npm run typec
• Unit Tests: npm run test:js completed successfully with all 1601/1601 Vitest tests passing.

Screenshots

Before Fix

After Fix

[AllTerrainDeveloper]

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