Agent rename dialog made compact #236

The rename-agent modal no longer inherits full-height flex growth; it is now a compact 520px-wide, content-height dialog centered by showModal.

Root cause was the inherited .key-dialog-body { flex: 1 }, which expanded the identity dialog to its maximum viewport height despite height:auto. Commit b0ebc7a sets height:fit-content and overrides the body to flex:0 1 auto. Desktop modal rendering was visually verified with Edge headless; all 85 tests pass. Not deployed yet.