Give a dashboard the panel's own size, and put the dots on its grid

A dashboard now carries the canvas it is drawn for (canvas_width /
canvas_height, presets plus two numbers in the settings panel). Editor and
wall panel render that surface at its true pixel size and scale it to fit,
so a side panel opening changes only the scale — never the arrangement
being made. With the width and column count known the dot pitch is exact,
(width + gap) / columns by row height + gap, so a dot sits where every
widget corner snaps. The wall panel shows no dots.

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

backend/app/flow/dashboards.py

@@ -180,6 +180,12 @@ class DashboardDef(BaseModel):
     #: 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)
+    #: The panel this dashboard is drawn for, in CSS pixels. Both the editor
+    #: and the wall panel scale that surface to fit whatever room they have, so
+    #: an arrangement does not depend on the window it was made in. Zero means
+    #: "unset" and the client falls back to its default.
+    canvas_width: int = Field(default=1920, ge=0, le=7680)
+    canvas_height: int = Field(default=1080, ge=0, le=4320)
     pages: list[PageDef] = Field(default_factory=list)
     #: Bumped on every save; a save based on an older one is refused.
     version: int = 1