Give the title bar strip back its drag, and the rule its full height

The window's content runs the full height underneath the title bar, so
the shell covers the bar - and a webview covering the bar swallows the
gesture that moves the window. The strip standing in for it now says it
is a drag region.

That strip also moves inside each column instead of spanning both, so the
rule between the nav and the app runs the whole height of the window
rather than starting below the bar.

docs/superpowers/specs/2026-09-01-work-app-design.md

@@ -281,7 +281,14 @@ load — falls back to its initial in a tile of the same size, so the row never
 ## The window
 
 The window keeps its ordinary macOS title bar, with the traffic lights where every other
-window puts them. It follows the app's own Light or Dark choice through `set_theme`;
+window puts them. Its content runs the full height *underneath* that bar, so each column
+carries its own strip standing in for it — the nav's and the app's — rather than one
+across the top. Two things follow from that, and both were bugs first:
+
+- Those strips are **drag regions**. They cover the title bar, and a webview that covers
+  the title bar swallows the gesture that moves the window.
+- They live **inside** each column rather than above both, so the rule between the nav and
+  the app runs the whole height of the window instead of starting below the bar. It follows the app's own Light or Dark choice through `set_theme`;
 "System" hands it back to the OS, which is what System means.
 
 An earlier version drew its own frame instead — a hidden title bar, a margin to grab, and