feat: Dynamic Input overlay (DYNMODE / F12)

- overlay.rs: dynamic_input_overlay() canvas widget draws a semi-transparent
  coordinate tooltip near the cursor; shows X/Y in absolute mode or
  dist/angle relative to last_point when a command base point exists
- statusbar: DYN toggle pill with F12 tooltip (calls ToggleDynInput)
- app/mod.rs: dyn_input: bool field (default true), ToggleDynInput message
- view.rs: builds DYN label and pushes overlay into viewport_stack when
  dyn_input=true and a command is active
- document.rs: last_cursor_screen: iced::Point — updated on every ViewportMove
- ROADMAP.md: Dynamic Input overlay marked ✅

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/app/update.rs

@@ -1031,6 +1031,7 @@ impl H7CAD {
                         pt
                     };
                     self.tabs[i].last_cursor_world = effective;
+                    self.tabs[i].last_cursor_screen = p;
 
                     let mut previews = if needs_entity {
                         let hover_handle =
@@ -1658,6 +1659,7 @@ impl H7CAD {
                 if self.polar_mode { self.ortho_mode = false; }
                 Task::none()
             }
+            Message::ToggleDynInput => { self.dyn_input ^= true; Task::none() }
             Message::SetPolarAngle(deg) => {
                 self.polar_increment_deg = deg;
                 self.polar_mode = true;