fix(ribbon): highlight follows each tool's mechanism; start-tab dimming (#355)

Tool highlights used to stick: every click set active_tool, but only
interactive commands and a hand-maintained per-modal list ever cleared
it, so one-shots (Cleanup/AUDIT, view actions) and unlisted dialogs
(Customization/ALIASEDIT, CUI, Plugin Manager…) stayed blue forever.

Now the rule is mechanical: state toggles light from state only (the
click highlight drops immediately), interactive commands stay lit
while running, dialog openers stay lit until the modal closes (generic
clear replaces the per-modal list), and one-shots never keep it — the
dispatch site turns the highlight off when nothing stayed running.

On the Start tab, tools whose command the start gate refuses now render
dimmed (grey icon + label) across all button styles, dropdowns and the
quick-access strip; start_allowed() is the single shared authority.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

src/app/view/mod.rs

@@ -1304,6 +1304,7 @@ impl OpenCADStudio {
             if !self.clean_screen {
                 col = col.push(self.ribbon.view(
                     is_paper,
+                    self.tabs[self.active_tab].is_start,
                     self.tabs[self.active_tab].history.undo_stack.len(),
                     self.tabs[self.active_tab].history.redo_stack.len(),
                 ));