style(workspace-notes): unify editor chrome, distinguish rail

- Drop border-bottom lines between header / title / tags / toolbar
  so they read as one continuous input surface.
- Add a single border-top above the editor body so the body's
  existing boundary stays clear.
- Notes rail gets --color-bg-card so it no longer visually merges
  with the tasks panel (both previously shared --color-surface).
  Dropped the rail's border-right since the tint now carries the
  division from the editor pane.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

frontend/src/components/WorkspaceNoteEditor.vue

@@ -462,7 +462,7 @@ defineExpose({ reload: loadProjectNotes });
   display: flex;
   flex-direction: column;
   overflow: hidden;
-  border-right: 1px solid var(--color-border);
+  background: var(--color-bg-card, var(--color-bg-secondary));
 }
 
 .rail-header {
@@ -715,7 +715,6 @@ defineExpose({ reload: loadProjectNotes });
   display: flex;
   align-items: center;
   padding: 0.5rem 0.75rem;
-  border-bottom: 1px solid var(--color-border);
   flex-shrink: 0;
   gap: 0.5rem;
 }
@@ -743,7 +742,6 @@ defineExpose({ reload: loadProjectNotes });
 
 .note-title-row {
   padding: 0.5rem 0.75rem;
-  border-bottom: 1px solid var(--color-border);
   flex-shrink: 0;
 }
 
@@ -763,7 +761,6 @@ defineExpose({ reload: loadProjectNotes });
   align-items: flex-start;
   gap: 0.4rem;
   padding: 0.4rem 0.6rem;
-  border-bottom: 1px solid var(--color-border);
   flex-shrink: 0;
 }
 .tag-row > :first-child { flex: 1; min-width: 0; }
@@ -789,7 +786,6 @@ defineExpose({ reload: loadProjectNotes });
   align-items: center;
   gap: 0.3rem;
   padding: 0.35rem 0.6rem;
-  border-bottom: 1px solid var(--color-border);
   background: color-mix(in srgb, var(--color-primary) 5%, var(--color-surface));
   flex-shrink: 0;
 }
@@ -824,7 +820,6 @@ defineExpose({ reload: loadProjectNotes });
 
 .toolbar-row {
   padding: 0.3rem 0.6rem;
-  border-bottom: 1px solid var(--color-border);
   flex-shrink: 0;
 }
 
@@ -834,7 +829,6 @@ defineExpose({ reload: loadProjectNotes });
   flex-wrap: wrap;
   gap: 0.25rem;
   padding: 0.3rem 0.6rem;
-  border-bottom: 1px solid var(--color-border);
   background: color-mix(in srgb, var(--color-primary) 5%, var(--color-surface));
   flex-shrink: 0;
 }
@@ -877,5 +871,6 @@ defineExpose({ reload: loadProjectNotes });
   flex: 1;
   overflow-y: auto;
   padding: 0.5rem 0.6rem;
+  border-top: 1px solid var(--color-border);
 }
 </style>