feat: make ui.panel flex align-items start by default (#604)

ui.panels implicitly wrap things in a flex, with a direction set to
column. Column makes sense, as tables typically are wide, and stacking
them makes more sense by default that side by side.

However, a key distinction is with row, the horizonal axis alignment is
equivalent to start. In column the horizonal axis (which is now a
cross-axis) is stretch.

I think we should set align-items: start for the implicit ui.flex column
wrapper of ui.panel so that it doesn't stretch buttons in the horizontal
cross-axis by default.

before:

![image](https://github.com/deephaven/deephaven-plugins/assets/1576283/8efc1b3a-0d6b-45ef-9f72-bdf9d73f43b9)

after:

![image_720](https://github.com/deephaven/deephaven-plugins/assets/1576283/7ce32114-238d-4c86-9293-d2b8faa776dc)

BREAKING CHANGE:

`ui.panel` implicitly created flexbox `align-items` property is now set
to `start` by default instead of `normal` (which is equivalent to
`stretch`). This results in a more pleasing default layout, as buttons
will no longer be stretched to the full panel width. You can match
previous behavior by explicitly returning a ui.panel with
`align-items="normal"` if required.

plugins/ui/src/deephaven/ui/components/panel.py

@@ -20,7 +20,7 @@ def panel(
     wrap: Wrap | None = None,
     justify_content: JustifyContent | None = None,
     align_content: AlignContent | None = None,
-    align_items: AlignItems | None = None,
+    align_items: AlignItems | None = "start",
     gap: DimensionValue | None = "size-100",
     column_gap: DimensionValue | None = None,
     row_gap: DimensionValue | None = None,

plugins/ui/src/js/src/layout/ReactPanel.tsx

@@ -65,7 +65,7 @@ function ReactPanel({
   wrap,
   justifyContent,
   alignContent,
-  alignItems,
+  alignItems = 'start',
   gap = 'size-100',
   rowGap,
   columnGap,

tests/app.d/ui_render_all.py

@@ -134,7 +134,12 @@ def ui_html_elements():
     ui.stack(
         ui.panel(
             ui.table(_column_types),
-            ui.grid(_my_components, _my_html_elements, columns=["1fr", "1fr", "1fr"]),
+            ui.grid(
+                _my_components,
+                _my_html_elements,
+                columns=["1fr", "1fr", "1fr"],
+                width="100%",
+            ),
             title="Panel B",
         ),
     )