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/store/layout.ts

@@ -23,6 +23,7 @@ export const SIDEBAR_SESSIONS_PAGE_SIZE = 50
 const SIDEBAR_PINNED_STORAGE_KEY = 'hermes.desktop.pinnedSessions'
 const SIDEBAR_AGENTS_GROUPED_STORAGE_KEY = 'hermes.desktop.agentsGroupedByWorkspace'
 const SIDEBAR_CRON_OPEN_STORAGE_KEY = 'hermes.desktop.sidebarCronOpen'
+const SIDEBAR_MESSAGING_OPEN_STORAGE_KEY = 'hermes.desktop.sidebarMessagingOpen'
 const SIDEBAR_SESSION_ORDER_STORAGE_KEY = 'hermes.desktop.sessionOrder'
 const SIDEBAR_WORKSPACE_ORDER_STORAGE_KEY = 'hermes.desktop.workspaceOrder'
 const PANES_FLIPPED_STORAGE_KEY = 'hermes.desktop.panesFlipped'
@@ -68,6 +69,10 @@ export const $sidebarRecentsOpen = atom(true)
 // default (it only renders at all when cron sessions exist) so the
 // scheduler's `[IMPORTANT: …]` first-message previews don't spam recents.
 export const $sidebarCronOpen = atom(storedBoolean(SIDEBAR_CRON_OPEN_STORAGE_KEY, false))
+// Messaging platform sections collapse by default (they can be numerous and
+// tall). We persist the ids the user has *explicitly expanded*, so the default
+// stays collapsed unless they've opened a platform before.
+export const $sidebarMessagingOpenIds = atom<string[]>(storedStringArray(SIDEBAR_MESSAGING_OPEN_STORAGE_KEY))
 export const $sidebarAgentsGrouped = atom(storedBoolean(SIDEBAR_AGENTS_GROUPED_STORAGE_KEY, false))
 // When true, the sessions sidebar moves to the right and the file browser +
 // preview rail move to the left — a mirror of the default layout.
@@ -77,6 +82,7 @@ export const $sessionsLimit = atom(SIDEBAR_SESSIONS_PAGE_SIZE)
 
 $pinnedSessionIds.subscribe(ids => persistStringArray(SIDEBAR_PINNED_STORAGE_KEY, [...ids]))
 $sidebarCronOpen.subscribe(open => persistBoolean(SIDEBAR_CRON_OPEN_STORAGE_KEY, open))
+$sidebarMessagingOpenIds.subscribe(ids => persistStringArray(SIDEBAR_MESSAGING_OPEN_STORAGE_KEY, [...ids]))
 $sidebarSessionOrderIds.subscribe(ids => persistStringArray(SIDEBAR_SESSION_ORDER_STORAGE_KEY, [...ids]))
 $sidebarWorkspaceOrderIds.subscribe(ids => persistStringArray(SIDEBAR_WORKSPACE_ORDER_STORAGE_KEY, [...ids]))
 $sidebarAgentsGrouped.subscribe(grouped => persistBoolean(SIDEBAR_AGENTS_GROUPED_STORAGE_KEY, grouped))
@@ -139,6 +145,14 @@ export function setSidebarCronOpen(open: boolean) {
   $sidebarCronOpen.set(open)
 }
 
+export function toggleSidebarMessagingOpen(sourceId: string) {
+  const current = $sidebarMessagingOpenIds.get()
+
+  $sidebarMessagingOpenIds.set(
+    current.includes(sourceId) ? current.filter(id => id !== sourceId) : [...current, sourceId]
+  )
+}
+
 export function setSidebarAgentsGrouped(grouped: boolean) {
   $sidebarAgentsGrouped.set(grouped)
 }