FN-6034: fix mobile workflow canvas width and project controls

Keep workflow editing usable on mobile while preserving related dashboard controls.

- allow the standalone project selector to expose a Manage Projects action in single-project layouts
- improve the mailbox resize handle contrast for better visibility
- make the workflow editor canvas and React Flow surface shrink and fill correctly without mobile horizontal overflow
- add CSS contract coverage for desktop and mobile workflow editor width behavior

Files changed:
 packages/dashboard/app/components/Header.tsx       |  4 +++-
 packages/dashboard/app/components/MailboxModal.css |  2 +-
 .../dashboard/app/components/ProjectSelector.tsx   | 15 +++--
 .../app/components/WorkflowNodeEditor.css          | 28 +++++++++
 .../__tests__/WorkflowNodeEditor.css.test.ts       | 66 ++++++++++++++++++++++
 5 files changed, 109 insertions(+), 6 deletions(-)

Fusion-Task-Id: FN-6034

Fusion-Task-Lineage: e7a784ff-188a-4538-a2be-f75484f10aa4

packages/dashboard/app/components/WorkflowNodeEditor.css

@@ -73,6 +73,7 @@
 .wf-editor-body {
   display: flex;
   flex: 1;
+  min-width: 0;
   min-height: 0;
 }
 
@@ -537,7 +538,10 @@
 
 .wf-editor-canvas {
   flex: 1;
+  width: 100%;
+  min-width: 0;
   min-height: 0;
+  overflow: hidden;
   position: relative;
 }
 
@@ -1230,6 +1234,9 @@
 
   .wf-editor-body {
     flex-direction: column;
+    width: 100%;
+    min-width: 0;
+    overflow-x: hidden;
   }
 
   .wf-editor-body--list-stage .wf-editor-sidebar {
@@ -1254,7 +1261,10 @@
   .wf-editor-body--editor-stage .wf-editor-canvas-wrap {
     display: flex;
     flex: 1 1 auto;
+    width: 100%;
+    min-width: 0;
     min-height: 0;
+    overflow: hidden;
   }
 
   .wf-editor-body--editor-stage .wf-editor-canvas {
@@ -1305,9 +1315,27 @@
 
   .wf-editor-canvas-wrap {
     flex: 1 1 auto;
+    width: 100%;
+    min-width: 0;
     min-height: 0;
   }
 
+  .wf-editor-canvas {
+    width: 100%;
+    min-width: 0;
+    max-width: 100%;
+    overflow: hidden;
+  }
+
+  .wf-editor-canvas .react-flow,
+  .wf-editor-canvas .react-flow__renderer,
+  .wf-editor-canvas .react-flow__pane,
+  .wf-editor-canvas .react-flow__viewport {
+    width: 100%;
+    min-width: 0;
+    max-width: 100%;
+  }
+
   .wf-editor-list-item,
   .wf-editor-new,
   .wf-editor-import,