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

backend/app/flow/dashboards.py

@@ -17,7 +17,7 @@ import threading
 from pathlib import Path
 from typing import Any, Literal
 
-from pydantic import BaseModel, Field, field_validator
+from pydantic import BaseModel, Field, field_validator, model_validator
 
 from app.flow.schemas import _validate_name
 from app.flow.store import FlowStore, StaleVersion
@@ -48,6 +48,18 @@ WidgetType = Literal[
 
 INPUT_WIDGETS = {"button", "switch", "slider", "input", "dropdown"}
 
+#: What a widget may be pointed at, by payload type. A switch that reads a
+#: float has nothing to show and nothing safe to send, so the pairing belongs
+#: to the document rather than to the editor that happened to write it. Types
+#: missing here take anything. Mirrored in the client
+#: (``frontend/src/components/Dashboard/widgets.tsx``).
+WIDGET_DTYPES: dict[str, set[str]] = {
+    "gauge": {"float", "int"},
+    "chart": {"float", "int"},
+    "slider": {"float", "int"},
+    "switch": {"bool"},
+}
+
 
 class Placement(BaseModel):
     """Where a widget sits in its section's grid, in grid units."""
@@ -104,6 +116,33 @@ class WidgetDef(BaseModel):
         points = int((self.config.get("history") or {}).get("points") or 0)
         return min(points, HISTORY_CAP)
 
+    @property
+    def bound_dtypes(self) -> list[str]:
+        """The payload types this widget was bound to, as the editor recorded.
+
+        Empty for a document written before the editor kept them, which is why
+        a missing type is never an error.
+        """
+        if self.type == "chart":
+            return [
+                str(series.get("dtype") or "")
+                for series in self.config.get("series") or []
+            ]
+        return [str(self.config.get("dtype") or "")]
+
+    @model_validator(mode="after")
+    def _check_binding(self) -> WidgetDef:
+        """Refuse a widget wired to a message it cannot carry."""
+        allowed = WIDGET_DTYPES.get(self.type)
+        if not allowed:
+            return self
+        for dtype in self.bound_dtypes:
+            if dtype and dtype not in allowed:
+                raise ValueError(
+                    f"a '{self.type}' widget cannot carry a '{dtype}' message"
+                )
+        return self
+
 
 class SectionDef(BaseModel):
     """A grid of widgets under a heading."""
@@ -138,6 +177,9 @@ class DashboardDef(BaseModel):
 
     name: str
     title: str = ""
+    #: How many columns the grid is cut into, so a dashboard can be matched to
+    #: the panel it will hang on.
+    columns: int = Field(default=12, ge=1, le=48)
     pages: list[PageDef] = Field(default_factory=list)
     #: Bumped on every save; a save based on an older one is refused.
     version: int = 1
@@ -335,6 +377,7 @@ __all__ = [
     "DASHBOARD_DIR",
     "HISTORY_CAP",
     "INPUT_WIDGETS",
+    "WIDGET_DTYPES",
     "DashboardDef",
     "DashboardExists",
     "DashboardNotFound",