feat(editor): empty-graph CTA + fit-to-screen button

Two polishing tweaks on the graph tab:

 - When the active flow has no steps, the canvas would have
   rendered as a near-empty grid (just inputs / outputs
   sidebars). Overlay a centred call-to-action with the
   graph icon, the "No steps yet" / "Noch keine Schritte"
   heading, a one-liner explainer, and a primary Add Step
   button that opens the capability picker directly.
 - Auto-fit zoom on first open of each flow + a floating
   "Fit to screen" button (bottom-right corner of the
   canvas viewport). The auto-fit re-runs only when the
   flow name changes, so the operator's manual pan / zoom
   on the current flow is preserved. The fit math expands
   the bounding box to include inputs + outputs pseudo-
   nodes too, so wide flows zoom out enough to show every
   port at once.

Fit math:
 - Walk every step's stored position + height; merge with
   the inputs sidebar's known position.
 - Compute scale that makes the bounding box fit the
   viewport with 80 px padding on each side.
 - Clamp to [0.4, 2.0] — InteractiveViewer's own bounds.
 - Translate so the box centres in the viewport.

`flutter analyze` clean, all 11 tests still pass.

Signed-off-by: flemming-it <sf@flemming.it>

lib/src/flow_editor_page.dart

@@ -416,11 +416,58 @@ outputs:
     // When a step is selected, show the properties panel on
     // the right as a fixed sidebar. Selection is owned by
     // the controller and ChangeNotifier rebuilds drive the
-    // panel show / hide.
+    // panel show / hide. When the flow has no steps at all,
+    // overlay a call-to-action so the operator's first
+    // instinct is the right action rather than staring at an
+    // empty grid.
+    final hasSteps = _controller.graph.steps.isNotEmpty;
     return Row(
       crossAxisAlignment: CrossAxisAlignment.stretch,
       children: [
-        Expanded(child: FlowCanvas(controller: _controller)),
+        Expanded(
+          child: Stack(
+            children: [
+              FlowCanvas(controller: _controller),
+              if (!hasSteps)
+                Positioned.fill(
+                  child: Container(
+                    color: theme.colorScheme.surface.withValues(alpha: 0.92),
+                    child: Center(
+                      child: Column(
+                        mainAxisSize: MainAxisSize.min,
+                        children: [
+                          Icon(
+                            Icons.account_tree_outlined,
+                            size: 48,
+                            color: theme.colorScheme.onSurfaceVariant,
+                          ),
+                          const SizedBox(height: FaiSpace.md),
+                          Text(
+                            _l.graphEmptyTitle,
+                            style: theme.textTheme.titleMedium,
+                          ),
+                          const SizedBox(height: FaiSpace.sm),
+                          Text(
+                            _l.graphEmptyBody,
+                            textAlign: TextAlign.center,
+                            style: theme.textTheme.bodySmall?.copyWith(
+                              color: theme.colorScheme.onSurfaceVariant,
+                            ),
+                          ),
+                          const SizedBox(height: FaiSpace.md),
+                          FilledButton.icon(
+                            onPressed: _addStep,
+                            icon: const Icon(Icons.add_box_outlined, size: 16),
+                            label: Text(_l.addStep),
+                          ),
+                        ],
+                      ),
+                    ),
+                  ),
+                ),
+            ],
+          ),
+        ),
         if (_controller.selectedStepId != null) ...[
           const VerticalDivider(width: 1),
           SizedBox(