Add the Fluksio look, and make it the one a dashboard starts in

Two looks were somebody else's language spoken well, and neither was the
product's. A dashboard nobody has dressed yet should look like the rest
of the app, so there is now a third set that follows the root
DESIGN-GUIDELINES.md to the letter — `--card` surfaces told from the page
by a hairline and a low shadow rather than by colour, every control a
pill, 16px panels, frosted floating chrome, one slate-blue accent spent
on what a person can act on — and it is what `look` means when nothing
says otherwise.

That also turns the exemption the other way round. The dashboard is still
allowed to look unlike the product; it just no longer does so by default.
An existing dashboard, which has never named a look, lands on the design
it had before any of this.

Restraint is the style rather than an omission here: no ripple, no glow,
no lift, and a press answered by the colour changing. The one deliberate
departure is the selector, which holds its choice in `--primary` rather
than the `--accent` the segmented rule asks for — that is a decision
about the widget, not about the look, and a control must not change what
it signals when the drawing changes. All three sets hold it the same way.

backend/fluksio/flow/dashboards.py

@@ -112,9 +112,9 @@ SETTING_DTYPES: dict[str, str] = {
     "theme": "str",
     # Read-only: the input widgets stop publishing.
     "locked": "bool",
-    # "glass" | "material": which of the two component sets draws this
-    # dashboard. Anything else is material. The two share every feature; only
-    # the drawing differs.
+    # "fluksio" | "material" | "glass": which component set draws this
+    # dashboard. Anything else is "fluksio", the app's own design. The three
+    # share every feature; only the drawing differs.
     "look": "str",
     # The dashboard's own colours, as an ordered list of hex strings. Position
     # is the role: background, surface, primary, accent, text, and anything