feat: Keyboard Shortcuts panel + SHORTCUTS command

Opens a reference panel listing all built-in keyboard shortcuts
(F3-F12, Ctrl+N/O/S/Z/C/X/V, etc.) and any user-defined overrides.

SHORTCUTS LIST         — open the panel
SHORTCUTS SET <k> <c>  — add/update a custom shortcut label
SHORTCUTS CLEAR <k>    — remove a custom entry

Custom shortcuts are stored in app state (runtime only) and shown
in the panel alongside the hardcoded bindings.

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

src/app/update.rs

@@ -2370,6 +2370,16 @@ impl H7CAD {
                 Task::none()
             }
 
+            // ── Keyboard Shortcuts Panel ──────────────────────────────────────
+            Message::ShortcutsPanelOpen => {
+                self.shortcuts_panel_open = true;
+                Task::none()
+            }
+            Message::ShortcutsPanelClose => {
+                self.shortcuts_panel_open = false;
+                Task::none()
+            }
+
             Message::ViewportContextMenuClose => {
                 let i = self.active_tab;
                 self.tabs[i].scene.selection.borrow_mut().context_menu = None;