Windows: resize border from the first frame, DPI-correct window moves (#1396)

* Windows: resize border from the first frame, DPI-correct window moves (#1381)

- The egui resize zones were laid out after the start page's early return, so a launch without
  a design had no resize edges at all. They now run on every page.
- Windows: the main window's procedure answers WM_NCHITTEST with the resize edges and corners,
  Windows' own border width at the window's DPI (8 px at 100 %), so the OS shows its resize
  cursors and runs its own resize (snap included) from the first message; no egui zones there.
- Windows: WM_GETDPISCALEDSIZE returns the size winit then applies (same logical size), so the
  rect the OS suggests while the title bar is dragged onto a monitor with another scale is the
  rect the window gets.
- Zone sizes and hit test shared in titlebar.rs and unit-tested; Windows CI checks the hit test
  right after launch and a real edge drag.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Windows resize smoke: find the main window by its title

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

* Windows resize smoke: probe the maximized window at the screen's edge

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>

AGENTS.md

@@ -27,11 +27,13 @@ new uses of it anywhere else. Public docs/web/Flatpak deployments use project Pa
 ## Never crash
 People trust SolidCraft with their designs; a crash loses their work. **This outranks feature work.**
 - **No panics in non-test code:** no `unwrap()`, `expect()`, `panic!`, `unreachable!`, `todo!`,
-  `unimplemented!`; no `unsafe` (`unsafe_code = "forbid"`). The two exceptions, in the app crate
+  `unimplemented!`; no `unsafe` (`unsafe_code = "forbid"`). The three exceptions, in the app crate
   (which denies it instead), have no safe API: two functions in
   `apps/solidcraft/src/open_documents.rs` (macOS hands Finder files only to an Objective-C
-  method) and the Windows-only `apps/solidcraft/src/native_crash.rs` (the exception filter that
-  records crashes inside native code such as graphics drivers).
+  method), the Windows-only `apps/solidcraft/src/native_crash.rs` (the exception filter that
+  records crashes inside native code such as graphics drivers) and the Windows-only
+  `apps/solidcraft/src/native_frame.rs` (the window procedure that answers the resize-border hit
+  test of the window without OS decorations).
 - **Errors are `Result<T, E>`** through the crate's error type and `?`. An unfinished feature
   returns an error ("not supported yet: …"); it never panics.
 - **Input-derived numbers are hostile** (command parameters, design files, control-channel