Rework the dashboards onto the flow canvas

One shell for both editors. Flows and dashboards each get a searchable
overview under the padded shell, their editors move to the full-bleed
canvas, and the floating chrome is shared: a title bar that only says
what you are looking at, and a bottom dock carrying everything else —
the flow bar's status, settings and Publish moved down there, the
add-flow button moved to the overview.

Dashboards gain the rest of M4's visualization work:

- widgets are picked by clicking them, with the header as the drag
  handle so a slider still slides and a switch still flips while
  editing; settings moved into the flows' SidePanel
- react-grid-layout for drag and edge-resize, so the stored x/y finally
  mean something; a dashboard nobody arranged is shelf-packed once
- a per-dashboard grid size, so a panel can be matched to its screen
- the chart widget, drawn with uPlot: several messages on one axis, fed
  from the stored history plus the live socket tail, coloured from the
  new --chart-1..5 ramp
- /view/{name}: the URL a wall panel is pointed at — no sidebar, no
  footer, no editing, and no editor code, since routes are split
- a widget wired to a payload type it cannot carry, or wired to nothing
  at all, carries the same red dot a failing node does; the picker
  records the type it bound and WidgetDef refuses a mismatch on save

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01H7LwYgJfpkbLCTeiAf8U4A

frontend/src/index.css

@@ -69,6 +69,11 @@
   --color-sidebar-border: var(--sidebar-border);
   --color-sidebar-ring: var(--sidebar-ring);
   --color-status-success: var(--status-success);
+  --color-chart-1: var(--chart-1);
+  --color-chart-2: var(--chart-2);
+  --color-chart-3: var(--chart-3);
+  --color-chart-4: var(--chart-4);
+  --color-chart-5: var(--chart-5);
   --font-mono:
     ui-monospace, "SF Mono", "Cascadia Code", "JetBrains Mono", Menlo, Consolas,
     monospace;
@@ -112,6 +117,11 @@
   --sidebar-border: #e4e4e4;
   --sidebar-ring: #4a7189;
   --status-success: #5e8b6d;
+  --chart-1: #3f5f74;
+  --chart-2: #4f768e;
+  --chart-3: #688ba1;
+  --chart-4: #83a2b5;
+  --chart-5: #9eb8c9;
 }
 
 .dark {
@@ -144,6 +154,11 @@
   --sidebar-border: #2a2a2a;
   --sidebar-ring: #7ba3b8;
   --status-success: #87b596;
+  --chart-1: #94b0be;
+  --chart-2: #7999aa;
+  --chart-3: #5f8396;
+  --chart-4: #496d7f;
+  --chart-5: #395767;
 }
 
 @layer base {