feat(app): add fixed Start tab with welcome page and recent documents

A pinned "Start" tab now lives at index 0. It cannot be closed, opens
on launch, and replaces the model-space viewport with a welcome page —
similar to Thunderbird's first-run screen.

- Boot opens with only the Start tab; Drawing1 is no longer auto-spawned.
- The Properties left panel becomes a Recent Documents rail on the
  Start tab. Recents persist to disk
  (%APPDATA%/H7CAD/recent.txt on Windows, $XDG_CONFIG_HOME/H7CAD/recent.txt
  on Linux, ~/Library/Application Support/H7CAD/recent.txt on macOS), up
  to 20 entries, saved on every open and rehydrated at boot.
  Each row opens or removes the file in place.
- The Support ribbon group (Donate / Report / About / Changelog) is
  retired; the same actions surface on the welcome page instead. The
  Donate button is the primary call-to-action — solid #B03020 brand
  fill with a soft brand-tinted ambient glow halo behind the action row
  (Thunderbird-style coloured glow against the neutral page).
- Drawing-only overlays (mouse-area, viewcube, navbar, dynamic input,
  selection markers) are suppressed on the Start tab so its clicks
  don't trigger CAD commands. OpenFile's "current_is_empty" check
  refuses to replace the Start tab, and TabClose ignores requests on
  it.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

src/app/update.rs

@@ -57,6 +57,27 @@ impl H7CAD {
 
             Message::OpenPathPicked(None) => Task::none(),
 
+            Message::OpenRecent(path) => {
+                // Recents are read from disk every save → the path may be
+                // stale. Skip silently if the file no longer exists; the
+                // entry stays in the list so the user can clean it up.
+                match std::fs::metadata(&path) {
+                    Ok(m) => self.update(Message::OpenPathPicked(Some((path, m.len())))),
+                    Err(_) => {
+                        self.command_line.push_error(&format!(
+                            "Recent file no longer exists: {}",
+                            path.display()
+                        ));
+                        Task::none()
+                    }
+                }
+            }
+
+            Message::RecentRemove(path) => {
+                self.app_menu.remove_recent(&path);
+                Task::none()
+            }
+
             Message::OpenPathPicked(Some((path, size_bytes))) => {
                 let name = path
                     .file_name()
@@ -107,7 +128,8 @@ impl H7CAD {
 
                 let current_is_empty = {
                     let t = &self.tabs[self.active_tab];
-                    t.current_path.is_none()
+                    !t.is_start
+                        && t.current_path.is_none()
                         && !t.dirty
                         && self.tabs[self.active_tab].scene.document.entities().count() == 0
                 };
@@ -754,13 +776,21 @@ impl H7CAD {
             }
 
             Message::TabClose(idx) => {
+                // Start tab is fixed — close requests on it are no-ops.
+                if self.tabs.get(idx).map_or(false, |t| t.is_start) {
+                    return Task::none();
+                }
                 if self.tabs.get(idx).map_or(false, |t| t.dirty) {
                     self.pending_close = Some(super::PendingClose::Tab(idx));
                     return self.open_unsaved_dialog_window();
                 }
+                // Only-tab case: when the lone non-start tab closes, fall
+                // back to the Start tab if it exists; otherwise spawn a
+                // fresh blank drawing (legacy behaviour).
                 if self.tabs.len() == 1 {
                     self.tab_counter += 1;
-                    self.tabs[0] = super::document::DocumentTab::new_drawing(self.tab_counter);
+                    self.tabs[0] =
+                        super::document::DocumentTab::new_drawing(self.tab_counter);
                     self.active_tab = 0;
                 } else {
                     self.tabs.remove(idx);