feat: add eslint-plugin-better-tailwindcss for Tailwind v4 linting (#9245)

## Summary

Add `eslint-plugin-better-tailwindcss` to the ESLint toolchain for
Tailwind CSS v4 class linting.

## Changes

- **What**: Integrate `eslint-plugin-better-tailwindcss` (v4.3.1) with
the recommended config, pointed at the design-system CSS entry point for
v4 theme resolution. Five rules are enabled initially:
`enforce-canonical-classes`, `no-deprecated-classes`,
`no-conflicting-classes`, `no-duplicate-classes`,
`no-unnecessary-whitespace`. Three rules are disabled pending follow-up:
`no-unknown-classes` (needs PrimeIcon/custom class whitelisting),
`enforce-consistent-line-wrapping` (oxfmt conflict risk),
`enforce-consistent-class-order` (large batch change).
- **Dependencies**: `eslint-plugin-better-tailwindcss` ^4.3.1
- Fix conflicting `outline outline-1` classes in
`FormDropdownMenuActions.vue` (caught by the new
`no-conflicting-classes` rule).

## Review Focus

- Is the rule severity/enablement strategy appropriate for incremental
adoption?
- The 700 warnings (mostly `enforce-canonical-classes` and
`no-deprecated-classes`) are all auto-fixable via `eslint --fix` —
should we batch-fix them in this PR or a follow-up?

Fixes COM-15518

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9245-feat-add-eslint-plugin-better-tailwindcss-for-Tailwind-v4-linting-3136d73d365081df8a64dd55962d073f)
by [Unito](https://www.unito.io)

---------

Co-authored-by: GitHub Action <action@github.com>
Co-authored-by: Alexander Brown <drjkl@comfy.org>

src/components/LiteGraphCanvasSplitterOverlay.vue

@@ -64,7 +64,7 @@
             layout="vertical"
             :pt:gutter="
               cn(
-                'rounded-tl-lg rounded-tr-lg ',
+                'rounded-tl-lg rounded-tr-lg',
                 !(bottomPanelVisible && !focusMode) && 'hidden'
               )
             "

src/components/topbar/CurrentUserButton.vue

@@ -12,7 +12,7 @@
         :class="
           cn(
             'flex items-center gap-1 rounded-full hover:bg-interface-button-hover-surface justify-center',
-            compact && 'size-full '
+            compact && 'size-full'
           )
         "
       >

src/components/widget/nav/NavTitle.vue

@@ -15,7 +15,7 @@
       v-if="collapsible"
       :class="
         cn(
-          'pi transition-transform duration-200 text-xs text-text-secondary ',
+          'pi transition-transform duration-200 text-xs text-text-secondary',
           isCollapsed ? 'pi-chevron-right' : 'pi-chevron-down'
         )
       "