Move window controls into the title bar

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

migrations/20260725150000_add_sidebar_state/down.sql

@@ -0,0 +1 @@
+ALTER TABLE preferences DROP COLUMN sidebar_collapsed;

migrations/20260725150000_add_sidebar_state/up.sql

@@ -0,0 +1,2 @@
+ALTER TABLE preferences ADD COLUMN sidebar_collapsed BOOLEAN NOT NULL DEFAULT FALSE
+    CHECK (sidebar_collapsed IN (FALSE, TRUE));