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/mod.rs

@@ -282,6 +282,11 @@ pub enum Message {
     /// File picker returned. `Some((path, size_in_bytes))` → start loading;
     /// `None` → user cancelled the dialog (no overlay shown).
     OpenPathPicked(Option<(PathBuf, u64)>),
+    /// Open a path from the Start tab's recent-documents list (skips the
+    /// file picker; the path is already known).
+    OpenRecent(PathBuf),
+    /// Drop a path from the recent-documents list.
+    RecentRemove(PathBuf),
     /// User clicked Cancel on the loading overlay. The parser thread keeps
     /// running but its result is discarded.
     OpenCancel,
@@ -688,14 +693,19 @@ pub enum Message {
 
 impl H7CAD {
     fn new() -> Self {
-        let first_tab = DocumentTab::new_drawing(1);
+        // Boot with only the Welcome/Start tab. The user creates drawings
+        // explicitly (File → New); we never auto-spawn Drawing1.
+        let start_tab = DocumentTab::new_start();
+        let mut app_menu = AppMenu::new();
+        // Restore recents from disk so the Start page lists them across runs.
+        app_menu.load_persistent_recents();
         let mut app = Self {
             start: Instant::now(),
-            tabs: vec![first_tab],
+            tabs: vec![start_tab],
             active_tab: 0,
-            tab_counter: 1,
+            tab_counter: 0,
             ribbon: Ribbon::new(),
-            app_menu: AppMenu::new(),
+            app_menu,
             command_line: CommandLine::new(),
             status_bar: StatusBar::new(),
             cursor_pos: Point::ORIGIN,