fix(desktop): sidebar sections never overlap — two-mode CSS scroll + collapse/cap groups (#43147)

* fix(desktop): prevent sidebar section overlap

Use a shared sidebar section scroller only on short windows so sections do not overlap, while preserving per-section scrolling on taller layouts.

* fix(desktop): measure section stack for compact sidebar mode

Window-height media query kept big windows in compact mode whenever the OS chrome ate into 830px; observe the section stack element instead so compact only engages when the stack is actually short.

* refactor(desktop): drive sidebar compact mode with CSS, not JS

Replace the matchMedia hook with a `short` (max-height: 830px) Tailwind
variant so the per-section scrollers flatten into one shared scroll stack on
short windows purely in CSS. Taller windows keep their per-group scrollers and
recents virtualization unchanged.

* refactor(desktop): pure-CSS two-mode sidebar scroll + collapse/cap groups

Drop the JS-measured compaction in favour of a single `compact` height
variant (max-height: 768px):
- tall: every section is its own capped, independent scroller; Sessions
  is the lone flex-1 scroller.
- short: sections flatten and the stack scrolls as one.

Every section is now `shrink-0`, so nothing is squeezed below its
content and bled onto a sibling — the root cause of the header overlap
(flexbox implied min-size). Sessions keeps its virtualized scroller in
short mode only when it's the long list.

Non-session groups (messaging, cron) collapse by default — expanded ids
persist per platform — and render 3 rows, revealing 10 more on demand.
Extract the shared SidebarLoadMoreRow. Stress harness seeds 50 recents
to mirror the real first page.

* chore(desktop): trim sidebar comments, unify "compact" naming

Self-review polish: condense the over-long mode comments, use "compact"
consistently (matching the variant) instead of mixing "short", and drop a
no-op useCallback around revealMoreMessaging.

* chore(desktop): drop dev sidebar stress harness from the PR

Remove stress-probe.ts and its main.tsx import — it was a throwaway
testing aid, not something to ship.

apps/desktop/src/styles.css

@@ -5,6 +5,10 @@
 @import '@vscode/codicons/dist/codicon.css';
 @custom-variant dark (&:is(.dark *));
 
+/* Sidebar sections: tall viewports give each its own scroller; compact ones
+   (this variant) flatten everything into one shared scroll. See ChatSidebar. */
+@custom-variant compact (@media (max-height: 768px));
+
 @font-face {
   font-family: 'Collapse';
   font-style: normal;
@@ -266,10 +270,12 @@
     --dt-user-bubble: var(--ui-chat-bubble-background);
     --dt-user-bubble-border: var(--ui-stroke-tertiary);
 
-    --dt-font-sans: 'Segoe WPC', 'Segoe UI', -apple-system, BlinkMacSystemFont, 'SF Pro Text', system-ui, sans-serif,
-      'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol', 'Noto Color Emoji', emoji;
-    --dt-font-mono: 'Cascadia Code', 'JetBrains Mono', 'SF Mono', ui-monospace, Menlo, Consolas, monospace,
+    --dt-font-sans:
+      'Segoe WPC', 'Segoe UI', -apple-system, BlinkMacSystemFont, 'SF Pro Text', system-ui, sans-serif,
       'Apple Color Emoji', 'Segoe UI Emoji', 'Segoe UI Symbol', 'Noto Color Emoji', emoji;
+    --dt-font-mono:
+      'Cascadia Code', 'JetBrains Mono', 'SF Mono', ui-monospace, Menlo, Consolas, monospace, 'Apple Color Emoji',
+      'Segoe UI Emoji', 'Segoe UI Symbol', 'Noto Color Emoji', emoji;
     --dt-base-size: 1rem;
     --dt-line-height: 1.5;
     --dt-letter-spacing: 0;
@@ -914,7 +920,11 @@ canvas {
 
   display: block;
   inline-size: var(--fit-available-space);
-  font-size: clamp(var(--fit-min, 1em), 1em * var(--fit-ratio), var(--fit-max, infinity * 1px) - var(--fit-support-sentinel));
+  font-size: clamp(
+    var(--fit-min, 1em),
+    1em * var(--fit-ratio),
+    var(--fit-max, infinity * 1px) - var(--fit-support-sentinel)
+  );
 }
 
 @container (inline-size > 0) {