Linux: prefer server DPI over panel physical DPI in vendored winit (#1150)

* fix(x11): prefer server DPI over panel physical DPI in winit

Without Xft DPI, use the X server screen dimensions before the output physical density. Xorg can run a dense panel at 96 DPI; EDID sizes alone make only winit apps appear zoomed.

Supersedes #908: preserves Xft.dpi priority, configured HiDPI screens, and WINIT_X11_SCALE_FACTOR override (including randr). Falls back to RandR output density when server dimensions are missing.

Adds default_dpi_factor() with 4 unit tests plus vendored-copy guard test in apps/pdfcraft/tests/winit_x11_dpi.rs.

* vendor/README: record the winit X11 DPI patch as (11)

---------

Co-authored-by: Brandon Thomas <echelon@gmail.com>

apps/pdfcraft/tests/winit_x11_dpi.rs

@@ -0,0 +1,14 @@
+#[test]
+fn winit_carries_the_x11_server_dpi_fallback() {
+    // The behavior tests live beside the policy in winit (util::randr::tests).
+    // This guards PdfCraft's vendored copy, which workspace tests do not test itself.
+    let root = std::path::Path::new(env!("CARGO_MANIFEST_DIR")).join("../..");
+    let randr = std::fs::read_to_string(root.join("vendor/winit/src/platform_impl/linux/x11/util/randr.rs")).unwrap();
+    assert!(randr.contains("let screen = self.default_root();"));
+    assert!(randr.contains("default_dpi_factor(\n                    self.get_xft_dpi(),"));
+    assert!(randr.contains("screen.width_in_millimeters.into()"));
+    assert!(randr.contains("calc_dpi_factor(screen_pixels, screen_mm)"));
+    assert!(randr.contains("server_dpi_wins_over_the_panels_physical_density"));
+    let desktop = std::fs::read_to_string(root.join("packaging/linux/ai.storyteller.pdfcraft.desktop")).unwrap();
+    assert!(!desktop.contains("WINIT_X11_SCALE_FACTOR"));
+}