fix(chrome): P5-47 — Mica vs frameless conflict broke custom window chrome

Root cause: lib.rs applies a Mica backdrop to the main window at startup;
set_custom_chrome then stripped the frame with set_decorations(false), and
Mica + frameless is a broken combination on Windows (DWM backdrop glitches
the whole surface).

- set_custom_chrome: clear_mica() before undecorating, re-apply_mica() when
  restoring the native frame; set_shadow(true) so the frameless window keeps
  its drop shadow + resize borders.
- window-state plugin: exclude StateFlags::DECORATIONS — the chrome toggle
  owns the decorated flag; restoring a saved decorated=false at startup would
  recreate the Mica-on-frameless glitch before the web side loads.

Pairs with the web-side TitleBar drag fix (explicit window_start_drag on
mousedown instead of data-tauri-drag-region) in the cinny repo.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src-tauri/src/lib.rs

@@ -411,7 +411,19 @@ pub fn run() {
             native::toast::show_rich_toast,
         ])
         .plugin(tauri_plugin_localhost::Builder::new(port).build())
-        .plugin(tauri_plugin_window_state::Builder::default().build())
+        .plugin(
+            // DECORATIONS is excluded: the custom-chrome toggle (set_custom_chrome)
+            // owns the decorated flag. Letting window-state restore a saved
+            // decorated=false at startup would re-create the frameless window
+            // BEFORE lib.rs applies Mica (a broken combination) and before the web
+            // side has pushed the user's current setting.
+            tauri_plugin_window_state::Builder::default()
+                .with_state_flags(
+                    tauri_plugin_window_state::StateFlags::all()
+                        & !tauri_plugin_window_state::StateFlags::DECORATIONS,
+                )
+                .build(),
+        )
         .plugin(tauri_plugin_opener::init())
         .plugin(tauri_plugin_notification::init())
         .plugin(tauri_plugin_deep_link::init());