fix(desktop): drop orphaned separators from sidebar section menus

[sumit-m]

What problem this solves

The sidebar section "more actions" menu (SectionActionsMenu) renders every item
conditionally, but two of its dividers are unconditional. When the items above a
divider all happen to be absent, the menu opens on a divider with nothing
before it.

The built-in Channels header hits this in normal use. AppSidebar supplies
it only two things that can produce items:

• onMarkAllRead, itself gated on hasUnread
• a sort preference (sortMode / onSortModeChange)

"Browse channels" is wired to the adjacent quick-create + button, not into this
menu, and rename/move/delete apply only to user-created sections. So the menu has
two states, and one of them is broken:

state	menu
a channel is unread	Mark all as read · divider · Sort — correct
everything read	divider · Sort — orphaned divider

That intermittency is why it reads as "sometimes broken": the divider is correct
right up until you read your last unread channel.

Fix

Compute whether anything actually precedes each divider and render it only then.

Two further points, same bug class, same component:

• The Delete section divider has the identical flaw. Reachable when a section
  supplies onDeleteSection but no rename/move handlers and no sort preference.
• showSectionManagement was gated on onRenameSection || onDeleteSection, but
  the block it guards renders rename/move-up/move-down. A delete-only section
  therefore opened the wrapper and emitted nothing — which would in turn make the
  new "is anything above me" check wrong. Now gated on the items it renders.

Verification

Confirmed by state transition in a running dev app rather than by inspection —
opened the Channels menu with unread present, clicked Mark all as read, then
reopened:

unread present  → items: [Mark all as read, separator, Sort]   separators: 1
all read        → items: [Sort]                                separators: 0

tsc --noEmit clean, biome clean, 3781 frontend unit tests pass.

Notes for reviewers

Not platform-specific — nothing here depends on the host, and it reproduces
anywhere the Channels section has no unread items.

Conflict likely with #2947
(sidebar categories and manual channel ordering), which also touches
CustomChannelSection.tsx and AppSidebar.tsx. That PR adds section
reordering, so it may introduce more conditional items above these dividers —
if it lands first, the hasItemsBeforeSort expression here needs its new
handlers added to the list. Happy to rebase behind it.