[feat] Improve group title layout (#9839)

Rebased and adopted from #5774 by @felixturner.

## Changes
- Remove unused font-size properties (`NODE_TEXT_SIZE`,
`NODE_SUBTEXT_SIZE`, `DEFAULT_GROUP_FONT`) from theme palettes and color
palette schema
- Replace `DEFAULT_GROUP_FONT`/`DEFAULT_GROUP_FONT_SIZE` with a single
`GROUP_TEXT_SIZE = 20` constant (reduced from 24px)
- Use `NODE_TITLE_HEIGHT` for group header height instead of `font_size
* 1.4`
- Vertically center group title text using `textBaseline = 'middle'`
- Use `GROUP_TEXT_SIZE` directly in TitleEditor instead of per-group
`font_size`
- Remove `font_size` from group serialization (no longer per-group
configurable)

## Original PR
Closes #5774

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9839-feat-Improve-group-title-layout-3216d73d36508112a0edc2a370af20ba)
by [Unito](https://www.unito.io)

---------

Co-authored-by: Felix Turner <felixturner@gmail.com>
Co-authored-by: github-actions <github-actions@github.com>
Co-authored-by: Alexander Brown <drjkl@comfy.org>

src/assets/palettes/github.json

@@ -34,9 +34,7 @@
       "CLEAR_BACKGROUND_COLOR": "#040506",
       "NODE_TITLE_COLOR": "#999",
       "NODE_SELECTED_TITLE_COLOR": "#e5eaf0",
-      "NODE_TEXT_SIZE": 14,
       "NODE_TEXT_COLOR": "#bcc2c8",
-      "NODE_SUBTEXT_SIZE": 12,
       "NODE_DEFAULT_COLOR": "#161b22",
       "NODE_DEFAULT_BGCOLOR": "#13171d",
       "NODE_DEFAULT_BOXCOLOR": "#30363d",
@@ -45,7 +43,6 @@
       "NODE_BYPASS_BGCOLOR": "#FF00FF",
       "NODE_ERROR_COLOUR": "#E00",
       "DEFAULT_SHADOW_COLOR": "rgba(0,0,0,0.5)",
-      "DEFAULT_GROUP_FONT": 24,
       "WIDGET_BGCOLOR": "#161b22",
       "WIDGET_OUTLINE_COLOR": "#30363d",
       "WIDGET_TEXT_COLOR": "#bcc2c8",