Add changelog entry for Cap Desktop 0.5.4

[Copilot]

Adds the missing changelog entry for the 0.5.4 desktop release, following the existing format used in 97.mdx and prior entries.

Changes

• Added apps/web/content/changelog/98.mdx with frontmatter (title, app, publishedAt, version) matching the cap-v0.5.4 tag.
• Summarized the changes shipped between cap-v0.5.3 and cap-v0.5.4, derived from the actual commit history:
  • Features: custom recordings storage path setting.
  • Fixes: Windows ghost overlay after recording, Windows screenshot clipboard white-background compositing, mic/video sync double-count bug, safer recordings folder error handling.
  • Performance: skipped unnecessary transparency pixel scan on file export.

Example

---
title: Custom Recordings Path & Windows Reliability Fixes
app: Cap Desktop
publishedAt: "2026-07-02"
version: 0.5.4
image:
---
...