claude: Add cross-mode logo fallback tests

This commit adds tests for how Quarto handles edge cases when logos are missing for
one theme mode but both light and dark modes are enabled:

1. dark-mode-no-dark-logo.qmd:
   Tests what happens when dark mode is enabled but only light logo exists

2. dark-mode-no-light-logo.qmd:
   Tests what happens when dark mode is enabled but only dark logo exists

3. document-light-brand-light-only.qmd:
   Tests document-level light-only logo + brand with light-only logo

4. document-dark-brand-dark-only.qmd:
   Tests document-level dark-only logo + brand with dark-only logo

Key learnings:
- Quarto implements cross-mode fallbacks where missing logos for one mode
  automatically use logos from the other mode
- Implementation in brand.ts shows the fallback logic applies in both directions:
  light → dark and dark → light
- Document-level logo settings take precedence over brand-level settings
- The fallback system ensures logos always appear regardless of which theme
  mode is active, providing a consistent user experience

Author: gordonwoodhull

tests/docs/smoke-all/brand/logo/dark-mode-no-dark-logo.qmd

@@ -0,0 +1,36 @@
+---
+title: Dark Mode with No Dark Logo
+format:
+  dashboard:
+    theme:
+      light: default
+      dark: darkly  # Explicitly enable dark mode
+brand:
+  color:
+    foreground: 
+      light: '#222'
+      dark: '#eee'
+    background:
+      light: '#eee'
+      dark: '#222'
+  logo:
+    images:
+      light-logo:
+        path: quarto.png
+        alt: light logo
+    medium:
+      light: light-logo
+      # No dark variant defined - what happens?
+_quarto:
+  tests:
+    dashboard:
+      ensureHtmlElements:
+        -
+          # Light mode should use the light logo
+          - 'img[src="quarto.png"][alt="light logo"][class="navbar-logo light-content d-inline-block"]'
+          # For dark mode, should it use the light logo since no dark logo exists?
+          - 'img[src="quarto.png"][alt="light logo"][class="navbar-logo dark-content d-inline-block"]'
+        - []
+---
+
+{{< lipsum 4 >}}

tests/docs/smoke-all/brand/logo/dark-mode-no-light-logo.qmd

@@ -0,0 +1,36 @@
+---
+title: Dark Mode with No Light Logo
+format:
+  dashboard:
+    theme:
+      light: default
+      dark: darkly  # Explicitly enable dark mode
+brand:
+  color:
+    foreground: 
+      light: '#222'
+      dark: '#eee'
+    background:
+      light: '#eee'
+      dark: '#222'
+  logo:
+    images:
+      dark-logo:
+        path: posit-logo-2024.svg
+        alt: dark logo
+    medium:
+      dark: dark-logo
+      # No light variant defined - what happens?
+_quarto:
+  tests:
+    dashboard:
+      ensureHtmlElements:
+        -
+          # Dark mode should use the dark logo
+          - 'img[src="posit-logo-2024.svg"][alt="dark logo"][class="navbar-logo dark-content d-inline-block"]'
+          # For light mode, should it use the dark logo since no light logo exists?
+          - 'img[src="posit-logo-2024.svg"][alt="dark logo"][class="navbar-logo light-content d-inline-block"]'
+        - []
+---
+
+{{< lipsum 4 >}}

tests/docs/smoke-all/brand/logo/document-dark-brand-dark-only.qmd

@@ -0,0 +1,39 @@
+---
+title: Document Dark Logo + Brand Dark Logo Only
+format:
+  dashboard:
+    theme:
+      light: default
+      dark: darkly  # Explicitly enable dark mode
+    # Document level logo with only dark defined
+    logo:
+      dark: direct-dark-logo.png
+brand:
+  color:
+    foreground: 
+      light: '#222'
+      dark: '#eee'
+    background:
+      light: '#eee'
+      dark: '#222'
+  logo:
+    images:
+      brand-dark-logo:
+        path: posit-logo-2024.svg
+        alt: brand dark logo
+    medium:
+      dark: brand-dark-logo
+      # No light variant defined in brand
+_quarto:
+  tests:
+    dashboard:
+      ensureHtmlElements:
+        -
+          # Dark mode should use document-level dark logo (not brand)
+          - 'img[src="direct-dark-logo.png"][alt=""][class="navbar-logo dark-content d-inline-block"]'
+          # For light mode, should use same document-level dark logo for fallback
+          - 'img[src="direct-dark-logo.png"][alt=""][class="navbar-logo light-content d-inline-block"]'
+        - []
+---
+
+{{< lipsum 4 >}}

tests/docs/smoke-all/brand/logo/document-light-brand-light-only.qmd

@@ -0,0 +1,39 @@
+---
+title: Document Light Logo + Brand Light Logo Only
+format:
+  dashboard:
+    theme:
+      light: default
+      dark: darkly  # Explicitly enable dark mode
+    # Document level logo with only light defined
+    logo:
+      light: direct-light-logo.png
+brand:
+  color:
+    foreground: 
+      light: '#222'
+      dark: '#eee'
+    background:
+      light: '#eee'
+      dark: '#222'
+  logo:
+    images:
+      brand-light-logo:
+        path: quarto.png
+        alt: brand light logo
+    medium:
+      light: brand-light-logo
+      # No dark variant defined in brand
+_quarto:
+  tests:
+    dashboard:
+      ensureHtmlElements:
+        -
+          # Light mode should use document-level light logo (not brand)
+          - 'img[src="direct-light-logo.png"][alt=""][class="navbar-logo light-content d-inline-block"]'
+          # For dark mode, should use same document-level light logo for fallback
+          - 'img[src="direct-light-logo.png"][alt=""][class="navbar-logo dark-content d-inline-block"]'
+        - []
+---
+
+{{< lipsum 4 >}}