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/document.rs

@@ -44,6 +44,11 @@ pub(super) struct DocumentTab {
     pub(super) active_mleader_style: String,
     /// Last camera_generation value written back to the document.
     pub(super) last_synced_camera_gen: u64,
+    /// Sentinel "Welcome / Start" tab. Always at index 0 when present.
+    /// Cannot be closed; the viewport area renders a welcome page instead
+    /// of the model-space shader. The scene is still constructed so the
+    /// rest of the code can treat it as a normal tab when reading.
+    pub(super) is_start: bool,
 }
 
 impl DocumentTab {
@@ -86,9 +91,20 @@ impl DocumentTab {
             refedit_session: None,
             active_mleader_style: "Standard".to_string(),
             last_synced_camera_gen: 0,
+            is_start: false,
         }
     }
 
+    /// Welcome / Start tab. Carries a dummy Scene so the rest of the app
+    /// can read tab state uniformly; the viewport renderer detects
+    /// `is_start` and shows a welcome page instead.
+    pub(super) fn new_start() -> Self {
+        let mut t = Self::new_drawing(0);
+        t.tab_title = "Start".to_string();
+        t.is_start = true;
+        t
+    }
+
     pub(super) fn tab_display_name(&self) -> String {
         match &self.current_path {
             Some(p) => p