refactor: restructure BaseModalLayout from flexbox to CSS Grid (#8211)

## Summary

Refactors `BaseModalLayout` from a flexbox-based layout to CSS Grid,
enabling smoother panel transitions and improved layout control.

## Changes

### Layout Restructure

- **Flexbox → CSS Grid**: Replaced nested flexbox with a 3-column CSS
Grid (`nav | main | aside`)
- **Smooth panel transitions**: Panel show/hide now animates via
`grid-template-columns` instead of Vue `<Transition>` with `translateX`
- **Removed transition CSS**: Deleted `.slide-panel-*` and `.fade-*`
transition styles (no longer needed with grid approach)

### Right Panel Improvements

- **Dedicated header**: Added header with close button, right panel
toggle, and customizable title slot (`rightPanelHeaderTitle`,
`rightPanelHeaderActions`)
- **New prop**: Added `rightPanelTitle` prop for simple text title in
right panel header

### UX & Accessibility

- **ESC key handling**: Pressing Escape closes the right panel (if open)
before closing the dialog
- **Accessibility**: Added `aria-label` attributes to all panel toggle
and close buttons
- **i18n**: Added translation keys: `showLeftPanel`, `hideLeftPanel`,
`showRightPanel`, `hideRightPanel`, `closeDialog`

---------

Co-authored-by: Amp <amp@ampcode.com>

browser_tests/tests/templates.spec.ts

@@ -189,9 +189,7 @@ test.describe('Templates', () => {
     const templateGrid = comfyPage.page.locator(
       '[data-testid="template-workflows-content"]'
     )
-    const nav = comfyPage.page
-      .locator('header')
-      .filter({ hasText: 'Templates' })
+    const nav = comfyPage.page.locator('header', { hasText: 'Templates' })
 
     await comfyPage.templates.waitForMinimumCardCount(1)
     await expect(templateGrid).toBeVisible()
@@ -201,7 +199,8 @@ test.describe('Templates', () => {
     await comfyPage.page.setViewportSize(mobileSize)
     await comfyPage.templates.waitForMinimumCardCount(1)
     await expect(templateGrid).toBeVisible()
-    await expect(nav).not.toBeVisible() // Nav should collapse at mobile size
+    // Nav header is clipped by overflow-hidden parent at mobile size
+    await expect(nav).not.toBeInViewport()
 
     const tabletSize = { width: 1024, height: 800 }
     await comfyPage.page.setViewportSize(tabletSize)