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/ui/overlay.rs

@@ -9,7 +9,7 @@ use iced::{Background, Border, Color, Element, Length, Point, Size, Theme};
 
 use crate::scene::object::GripShape;
 use crate::scene::SelectionState;
-use crate::Message;
+use crate::app::Message;
 
 /// Half-size of the crosshair center square in screen pixels (square = SQ*2 × SQ*2).
 pub const CROSSHAIR_SQ: f32 = 7.5;