Place apps below the title bar, not behind it

A child webview is positioned against the window frame while the shell
measures the hole it left from inside the content view. Borderless, those
origins coincided and it never showed. With a title bar back they are a
title bar apart, so every app was drawn that much too high - painting
over the right-hand part of the bar, which read as the bar being tinted
by whichever site was open, and leaving a strip of the same height along
the bottom.

Tauri cannot report that height. Both inner_position/outer_position and
inner_size/outer_size come back identical on macOS - measured, both
differences were zero against a window whose content is plainly a title
bar shorter than its frame. NSWindow.contentLayoutRect knows, so it is
asked once and cached.

Also hides the window title, and centres the navigation row in its own
strip rather than letting it crowd the bar above it.

src-tauri/tauri.conf.json

@@ -18,7 +18,8 @@
         "height": 900,
         "minWidth": 900,
         "minHeight": 600,
-        "center": true
+        "center": true,
+        "hiddenTitle": true
       }
     ],
     "security": { "csp": null }