fix(docs): improve mobile sidebar navigation

The sidebar had all categories expanded by default (collapsed: false),
which on mobile created a 60+ item flat list when opening the sidebar.
Reported by danny on Discord.

Changes:
- Set all top-level categories to collapsed: true (tap to expand)
- Enable autoCollapseCategories: true (accordion — opening one section
  closes others, prevents the overwhelming flat list)
- Enable hideable sidebar (swipe-to-dismiss on mobile)
- Add mobile CSS: larger touch targets (0.75rem padding), bolder
  category headers, visible subcategory indentation with left border,
  wider sidebar (85vw / 360px max), darker backdrop overlay

website/sidebars.ts

@@ -5,7 +5,7 @@ const sidebars: SidebarsConfig = {
     {
       type: 'category',
       label: 'Getting Started',
-      collapsed: false,
+      collapsed: true,
       items: [
         'getting-started/quickstart',
         'getting-started/installation',
@@ -17,7 +17,7 @@ const sidebars: SidebarsConfig = {
     {
       type: 'category',
       label: 'Guides & Tutorials',
-      collapsed: false,
+      collapsed: true,
       items: [
         'guides/tips',
         'guides/daily-briefing-bot',
@@ -32,7 +32,7 @@ const sidebars: SidebarsConfig = {
     {
       type: 'category',
       label: 'User Guide',
-      collapsed: false,
+      collapsed: true,
       items: [
         'user-guide/cli',
         'user-guide/configuration',