feat: integrate Coloris color picker for background field

Add Coloris.js library for enhanced color selection in PageSettings with
automatic button state sync across tab changes.

Features:
- Color picker with swatches, alpha support, and format toggle (hex/rgb/hsl)
- Button positioned to the left of input field
- Automatic sync when switching tabs (remembers selected color)
- Close button and click-outside-to-close functionality
- Dark theme with pill UI style

Changes:
- Install @melloware/coloris package
- PageSettings.vue: Integrate Coloris with data-coloris attribute,
  add tab visibility detection via provide/inject, force button update
  when returning to document tab
- EditorPanel.vue: Provide activeTab to child components, increase
  panel width to 30rem
- _forms.scss: Add .input-with-color styles with custom Coloris
  button positioning (absolute positioned to left of input)
- Temporarily comment out rgb/hex format buttons (replaced by Coloris
  format toggle)

Technical details:
- Uses provide/inject pattern to detect tab changes
- Triggers synthetic input events to force Coloris button refresh
- Custom CSS overrides to position swatch button correctly

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

public/assets/css/stylesheet.css

@@ -0,0 +1,24 @@
+@page {
+  size: A4;
+  margin: 20mm 15mm 26mm 15mm;
+}
+
+@page {
+  @bottom-center { content: string(title); }
+}
+
+h2 {
+  break-before: page;
+}
+
+.chapter > h2 {
+  string-set: title content(text);
+}
+
+#chapter-2 {
+  font-size: 2rem;
+}
+
+p {
+  font-size: 1rem;
+}