docs: add release notes for v0.1.0–v0.4.0, v0.6.0 and v0.7.0

Backfill notes for versions that had a tag but no release page, so
every version up to v0.7.1 has a GitHub/Gitea release. Tag v0.7.0 at
45b27d5 (was missing).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/release-notes/v0.2.1.md

@@ -0,0 +1,17 @@
+# v0.2.1 — Fix crash after dragging a part
+
+## 🐛 Bug fixes
+
+**No more crash after using drag mode**
+* Clicking anywhere in the view after dragging a part could crash the app. Fixed — you can drag, then measure or select as usual.
+
+## 📦 Downloads
+
+Source only — no prebuilt binaries were published for this version. For a ready-to-run build, grab the [latest release](https://github.com/ETWen/STPViewer/releases/latest).
+
+## 🔗 Links
+
+* [Architecture](https://github.com/ETWen/STPViewer/blob/main/ARCHITECTURE.md)
+* [README](https://github.com/ETWen/STPViewer/blob/main/README.md)
+
+**Full changelog:** [v0.2.0...v0.2.1](https://github.com/ETWen/STPViewer/compare/v0.2.0...v0.2.1)