feat(editor): three-tab WYSIWYG editor — graph / text / run

Full rewrite of the editor surface, layered on top of the
FlowGraph foundation. One in-memory flow drives three tabs
that the operator can flip between freely:

 - Graph: a drag-and-drop canvas. Nodes are step cards with
   port dots on their left (one per `with:` field) and a
   combined output port on the right. Pinned inputs and
   outputs pseudo-nodes sit at the left and right edges so
   every flow has a visually obvious source and sink. Pan +
   zoom via InteractiveViewer; drag a node by its body to
   reposition it (positions persisted to a sidecar JSON file
   under ~/.fai/data/flows/.layout/<name>.json — kept OUT of
   the YAML so `fai run` stays byte-stable).
 - Text: the existing YAML CodeField with expands:true so
   line 1 anchors at the top edge. YAML-aware syntax
   highlighting picks up the theme's primary / secondary /
   tertiary palette for keys / strings / numbers.
 - Run: an inputs form (text fields + file-pick), a Start
   button that calls the host's FlowRunDriver, a live step
   list driven by the driver's event stream (matches the
   `fai run` CLI rendering — ◻ pending, · running, ✔ done +
   duration, ✗ failed, ⏸ awaiting approval), and the typed
   outputs once the run resolves.

Source of truth = the YAML text. Graph edits emit fresh YAML
into the shared CodeController; text edits re-parse the
graph on a 350 ms debounce. Layout sidecar persists drag
positions only.

New public API (lib/fai_studio_flow_editor.dart):

  FlowEditorPage(
    initialFlowName: ...,
    locale: ...,
    runDriver: FlowRunDriver?,        // NEW — host bridge
    availableCapabilities: List<String>, // NEW — for the
                                          // capability picker
                                          // dialog when adding
                                          // a step
  )

The host (Studio) implements FlowRunDriver to bridge the
hub's gRPC SDK into the editor's event vocabulary. The
StepStarted/Completed/Failed/AwaitingApproval events are
shared verbatim with the CLI's run_progress renderer so
both surfaces speak the same visual language.

Files in this commit:
 - lib/src/editor_controller.dart       — shared state +
   debounced reparse loop
 - lib/src/run_driver.dart              — host bridge
   interface + event types
 - lib/src/widgets/flow_canvas.dart     — pan / zoom / drag /
   port-to-port connection drawing
 - lib/src/widgets/flow_node.dart       — node card primitive
   (module / approval / inputs / outputs variants)
 - lib/src/widgets/edge_painter.dart    — single CustomPainter
   for every edge + draft drag line, cubic bezier with
   arrow-head caps
 - lib/src/widgets/properties_panel.dart — right-side editor
   when a step is selected (rename id, change capability, add
   / remove / rename with-fields, delete step)
 - lib/src/widgets/capability_picker.dart — searchable list
   dialog used by Add-step
 - lib/src/widgets/run_tab.dart         — inputs form +
   live step progress + outputs renderer
 - lib/src/flow_editor_page.dart        — host scaffolding,
   toolbar, file list, three-tab body, keyboard shortcuts
 - lib/src/l10n.dart                    — EN + DE strings for
   every new label
 - lib/fai_studio_flow_editor.dart      — exports the new
   public types (FlowRunDriver, FlowRunEvent variants,
   FlowOutputValue variants)

flutter analyze: 0 issues. flutter test: 7/7 green.

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

lib/src/widgets.dart

@@ -29,11 +29,7 @@ class FaiEmptyState extends StatelessWidget {
           mainAxisSize: MainAxisSize.min,
           crossAxisAlignment: CrossAxisAlignment.center,
           children: [
-            Icon(
-              icon,
-              size: 40,
-              color: theme.colorScheme.onSurfaceVariant,
-            ),
+            Icon(icon, size: 40, color: theme.colorScheme.onSurfaceVariant),
             const SizedBox(height: FaiSpace.md),
             Text(
               title,
@@ -91,11 +87,7 @@ class FaiErrorBox extends StatelessWidget {
       child: SingleChildScrollView(
         child: SelectableText(
           error?.toString() ?? '',
-          style: TextStyle(
-            fontFamily: 'monospace',
-            fontSize: 12,
-            color: fg,
-          ),
+          style: TextStyle(fontFamily: 'monospace', fontSize: 12, color: fg),
         ),
       ),
     );