fix(KB-660): fix Git Manager dialog rendering off-screen on small viewports

- Fix .gm-content min-height from 400px to 0 for proper flex behavior
- Add mobile responsive styles to prevent modal overflow
- Create changeset for patch release

.changeset/fix-git-manager-modal.md

@@ -0,0 +1,8 @@
+---
+"@gsxdsm/fusion": patch
+---
+
+Fix Git Manager dialog rendering off-screen on smaller viewports
+
+Changed `.gm-content` min-height from 400px to 0, allowing the modal content
+to properly flex within the viewport instead of forcing overflow.