Implement multi-session support and welcome screen in EditorManager

- Removed the homepage display logic and replaced it with a welcome screen that appears when no ROM is loaded or no active editors are present.
- Enhanced session management by iterating through all sessions to check for active editors, allowing for better multi-session docking.
- Introduced new methods for generating unique editor titles based on session context and added feature flags for per-session configurations.
- Added safeguards for ROM loading state checks in editor methods to prevent operations on unloaded ROMs.

src/app/editor/overworld/overworld_editor.cc

@@ -164,6 +164,10 @@ void OverworldEditor::Initialize() {
 }
 
 absl::Status OverworldEditor::Load() {
+  if (!rom_ || !rom_->is_loaded()) {
+    return absl::FailedPreconditionError("ROM not loaded");
+  }
+  
   RETURN_IF_ERROR(LoadGraphics());
   RETURN_IF_ERROR(
       tile16_editor_.Initialize(tile16_blockset_bmp_, current_gfx_bmp_,

src/app/editor/overworld/overworld_editor.h

@@ -106,6 +106,14 @@ class OverworldEditor : public Editor, public gfx::GfxContext {
   int jump_to_tab() { return jump_to_tab_; }
   int jump_to_tab_ = -1;
 
+  // ROM state methods (from Editor base class)
+  bool IsRomLoaded() const override { return rom_ && rom_->is_loaded(); }
+  std::string GetRomStatus() const override {
+    if (!rom_) return "No ROM loaded";
+    if (!rom_->is_loaded()) return "ROM failed to load";
+    return absl::StrFormat("ROM loaded: %s", rom_->title());
+  }
+
   /**
    * @brief Load the Bitmap objects for each OverworldMap.
    *