feat(start): three-part start page (Recent · Welcome · Supporters) that tabs when narrow

Rebuild the Start page as three parts — Recent Files (left), Welcome
(center) and Supporters (right) — that sit side by side while they fit.
When the page is too narrow for all three it falls back to a tab bar
(Recent Files / Welcome / Supporters) showing one section at a time, with
Welcome open by default (new StartSection state + StartSectionSelect).

The Recent Documents list moves off the left properties slot and into the
page itself. The Welcome column's button rows now flex-wrap, and the two
intro-video cards sit side by side, wrap to a stacked column when the
width gets tight, then shrink to fit the remaining height so they never
spill over the status bar. Trimmed the headline to just "Open CAD Studio"
(dropped "Welcome to" + the subtitle) and the "Release Notes" button, and
tightened the page's side padding.

Co-Authored-By: Claude Opus 4.8 <[email protected]>

src/app/update/mod.rs

@@ -221,6 +221,11 @@ impl OpenCADStudio {
                 Task::none()
             }
 
+            Message::StartSectionSelect(section) => {
+                self.start_section = section;
+                Task::none()
+            }
+
             Message::ScrollLayoutTabs(dx) => iced::widget::operation::scroll_by(
                 iced::advanced::widget::Id::new(crate::ui::statusbar::LAYOUT_TABS_SCROLL_ID),
                 iced::widget::scrollable::AbsoluteOffset { x: dx, y: 0.0 },