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/ui/properties.rs

@@ -22,7 +22,7 @@ const COMBO_PAD_V: f32   = (ROW_H - FONT_SZ * 1.3 - 2.0) / 2.0;  // fills combo
 const SWATCH_SZ: f32     = ROW_H * 0.54;                          // ≈14 px color swatch
 
 use crate::scene::object::{PropSection, PropValue};
-use crate::Message;
+use crate::app::Message;
 
 const VARIES_LABEL: &str = "*VARIES*";