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 <noreply@anthropic.com>

src/app/document.rs

@@ -0,0 +1,83 @@
+use crate::scene::Scene;
+use crate::ui::{LayerPanel, PropertiesPanel};
+use crate::command::CadCommand;
+use crate::snap::SnapResult;
+use crate::scene::grip::GripEdit;
+use crate::scene::GripDef;
+use acadrust::{CadDocument, Handle};
+use crate::linetypes;
+use std::path::PathBuf;
+
+// ── Per-document tab state ─────────────────────────────────────────────────
+
+pub(super) struct DocumentTab {
+    pub(super) scene: Scene,
+    pub(super) current_path: Option<PathBuf>,
+    pub(super) dirty: bool,
+    pub(super) tab_title: String,
+    pub(super) properties: PropertiesPanel,
+    pub(super) layers: LayerPanel,
+    pub(super) active_cmd: Option<Box<dyn CadCommand>>,
+    pub(super) last_cmd: Option<String>,
+    pub(super) snap_result: Option<SnapResult>,
+    pub(super) active_grip: Option<GripEdit>,
+    pub(super) selected_grips: Vec<GripDef>,
+    pub(super) selected_handle: Option<Handle>,
+    pub(super) wireframe: bool,
+    pub(super) visual_style: String,
+    pub(super) last_cursor_world: glam::Vec3,
+    pub(super) history: HistoryState,
+    pub(super) active_layer: String,
+}
+
+impl DocumentTab {
+    pub(super) fn new_drawing(n: usize) -> Self {
+        let mut scene = Scene::new();
+        linetypes::populate_document(&mut scene.document);
+        Self {
+            scene,
+            current_path: None,
+            dirty: false,
+            tab_title: format!("Drawing{}", n),
+            properties: PropertiesPanel::empty(),
+            layers: LayerPanel::default(),
+            active_cmd: None,
+            last_cmd: None,
+            snap_result: None,
+            active_grip: None,
+            selected_grips: vec![],
+            selected_handle: None,
+            wireframe: false,
+            visual_style: "Shaded".into(),
+            last_cursor_world: glam::Vec3::ZERO,
+            history: HistoryState::default(),
+            active_layer: "0".to_string(),
+        }
+    }
+
+    pub(super) fn tab_display_name(&self) -> String {
+        match &self.current_path {
+            Some(p) => p
+                .file_name()
+                .unwrap_or_default()
+                .to_string_lossy()
+                .to_string(),
+            None => self.tab_title.clone(),
+        }
+    }
+}
+
+#[derive(Clone)]
+pub(super) struct HistorySnapshot {
+    pub(super) document: CadDocument,
+    pub(super) current_layout: String,
+    pub(super) selected: Vec<Handle>,
+    pub(super) dirty: bool,
+    pub(super) label: String,
+}
+
+#[derive(Default)]
+pub(super) struct HistoryState {
+    pub(super) undo_stack: Vec<HistorySnapshot>,
+    pub(super) redo_stack: Vec<HistorySnapshot>,
+}