feat(desktop): add user-defined channel sections to sidebar

[wpfleger96]

Adds Slack-style sidebar sections so users can organize stream channels into custom named groups — the flat channel list gets unwieldy as the channel count grows.

Users create sections via right-click on any stream channel ("Move to section → New section…") and manage them via right-click on the section header (rename, delete). Sections and channels are both drag-and-droppable: drag a channel onto a section to assign it, drag section headers to reorder them. The context menu retains move up/down and "Move to section" as keyboard-accessible alternatives. Each section collapses independently. Channels not assigned to a section remain in the default "Channels" group below all custom sections. Each section header includes a "mark all as read" button.

• channelSectionsStorage.ts — pure localStorage read/write keyed by sprout-channel-sections.v1:{pubkey} with parseChannelSectionPayload type-guard validation, stripOrphanedAssignments cleanup, and a safe default; sections and channel assignments stored separately for O(1) lookups
• useChannelSections.ts — hook owning all CRUD mutations (create, rename, delete, assign/unassign, reorderSections) with cross-tab sync via StorageEvent
• ChannelSectionDialogs.tsx — SectionNameDialog base with thin CreateSectionDialog/RenameSectionDialog wrappers using existing Dialog/Input primitives
• CustomChannelSection.tsx — ChannelGroupSection, SectionHeaderActions, CustomChannelSection, context menu items, and "Move to section" submenu; AppSidebar.tsx updated to drive section state and render custom sections above the default group
• SidebarDnd.tsx — all @dnd-kit primitives extracted here: SidebarDndContext (wraps DndContext + SortableContext + DragOverlay), SortableSectionShell, DraggableChannelRow, DroppableSectionBody, DroppableUngroupedBody, DragOverlayChannel, DragOverlaySection; uses pointerWithin collision detection to align drop zones with visual section boundaries
• channelSectionsHelpers.ts — pure swapSectionOrder function extracted for testability
• Fixes a pre-existing bug in useUnreadChannels.ts where markChannelRead silently failed when channel.lastMessageAt was null (missing latestByChannelRef fallback that markChannelUnread already had), and clears forcedContexts immediately in readStateManager.ts:markContextRead to prevent a 5s async window where relay merges are blocked

Stacked: cross-device sync added in #792

[wesbillman]

@wpfleger96 will this work across devices as well? Like will the mobile app have access to these same groups to display?

[wpfleger96]

@wesbillman no, as-is the PR will save channel sections to localStorage on each device

I'm gonna add Nostr event syncing so that sections will be shared between devices in a stacked PR so this one doesn't get too chonky

[wpfleger96]

Channel section creation:

1. 
2. 
3. 

[wpfleger96]

multiple sections:

section context menu:

[wpfleger96]

Click-and-drag channels in/out of sections:

[wpfleger96]

deleting a section returns its channels back to default Channels

[wpfleger96]

Click-and-drag section order