Refactor src/main.rs into modular src/app/ structure

Split the 4500-line main.rs into focused submodules under src/app/:
- mod.rs: H7CAD struct, Message enum, boot/new/run
- document.rs: DocumentTab, HistorySnapshot, HistoryState
- helpers.rs: pure helper functions
- history.rs: undo/redo logic
- properties.rs: properties panel logic
- layers.rs: layer sync
- commands.rs: dispatch_command
- cmd_result.rs: apply_cmd_result
- view.rs: view, subscription, tab bar
- update.rs: update message handler

Co-Authored-By: Claude Sonnet 4.6 <[email protected]>

src/app/layers.rs

@@ -0,0 +1,43 @@
+use super::H7CAD;
+use crate::ui;
+
+impl H7CAD {
+    pub(super) fn sync_ribbon_layers(&mut self) {
+        let i = self.active_tab;
+        let active = self.tabs[i].active_layer.clone();
+        let infos: Vec<crate::ui::ribbon::LayerInfo> = self.tabs[i]
+            .layers
+            .layers
+            .iter()
+            .map(|l| crate::ui::ribbon::LayerInfo {
+                name: l.name.clone(),
+                color: l.color,
+                visible: l.visible,
+                frozen: l.frozen,
+                locked: l.locked,
+            })
+            .collect();
+        let names: Vec<String> = infos.iter().map(|l| l.name.clone()).collect();
+        let active = if names.contains(&active) { active } else { "0".to_string() };
+        self.tabs[i].active_layer = active.clone();
+        self.tabs[i].layers.current_layer = active.clone();
+        self.ribbon.set_layers(infos, &active);
+        let lt_items: Vec<ui::properties::LinetypeItem> = self.tabs[i]
+            .scene
+            .document
+            .line_types
+            .iter()
+            .map(|lt| {
+                let name = if lt.name.eq_ignore_ascii_case("bylayer") {
+                    "ByLayer".to_string()
+                } else {
+                    lt.name.clone()
+                };
+                let art = crate::linetypes::extract_pattern(&lt.description);
+                ui::properties::LinetypeItem { name, art }
+            })
+            .collect();
+        self.tabs[i].layers.sync_linetypes(lt_items.clone());
+        self.ribbon.set_available_linetypes(lt_items);
+    }
+}