Polish feedback entry points (Linux About link + README/site) and prepare v0.2.4 #55

Declaring shared-layer work before writing (跨设备单写声明):

1. Linux host: aboutReportIssue i18n string is defined but unused — the GTK About dialog (gtk_about_dialog) cannot carry a second link, so macOS/Windows About pages have the issue-tracker link and Linux does not. Fix: rebuild show_about_dialog() on a plain GtkDialog with the same family layout (tagline, license, website + issue link buttons).
2. README.md / README.zh-CN.md: add a Feedback section pointing at GitHub issues (offline-first, no telemetry, issues are the channel).
3. docs site: add a footer feedback link (footer.feedback i18n, en/zh).
4. v0.2.4 release prep: fold the above into the changelog entry; release via the version-PR flow (see ci(release): automate generated clients and gated publishing #53).

Starting commit: c7ae32d. Linux-host verification is via CI (this is the macOS leg machine); the version bump itself will ride the release/version-v0.2.4 branch after #53 merges.