feat(desktop): keyboard shortcut to navigate to next/previous channel

[eddieberklee]

Problem

There's no way to move between channels from the keyboard. Every channel switch requires the mouse or a round-trip through quick search (⌘K) — typing a name you already know to reach a channel that's already visible in the sidebar. Slack, Discord, and IRC clients all have a single-chord next/previous-conversation binding, and its absence is felt immediately when working across a few adjacent channels.

Proposal

Add ⌥↓ / ⌥↑ (Ctrl+Alt+↓ / Ctrl+Alt+↑ on Windows) to move the active channel selection down/up the sidebar list.

• Order matches what the sidebar displays: starred → custom sections in section order → unassigned, each honoring its own sort preference (as assembled in AppSidebar.tsx:395-433).
• Scope: the active community's stream channels. Forums and DMs are out of scope here.
• Muted channels are skipped. mutedChannelIds is already available in AppShell, so this is a natural filter rather than a new data path.
• Stops at both ends rather than wrapping — no wraparound surprise when you're walking to the top or bottom of a list deliberately.
• No-op when the list is empty or no channel is selected (e.g. on the home feed).
• Registered in shared/lib/keyboard-shortcuts.ts under Navigation, so it appears in Settings → Keyboard Shortcuts automatically.

Windows uses Ctrl+Alt+arrows rather than Slack's plain Alt+arrows because Alt+←/Alt+→ are already bound to back/forward there (useBackForwardControls.ts:92-106).

Implementation sketch

The displayed-order flattening currently lives inline in AppSidebar.tsx. I'd extract it into a shared helper so the shortcut and the sidebar can't drift apart, cover it with a .test.mjs unit test (matching the convention in features/sidebar/lib/), add a useChannelNavigationShortcuts hook alongside the existing useMarkAsReadShortcuts, and add an e2e case to navigation.spec.ts.

Known limitation in this first pass

Sidebar collapse state (collapsedGroups / collapsedSections) is local React.useState inside AppSidebar and isn't reachable from an app-level shortcut hook. So navigating into a collapsed group will switch the channel correctly but leave the group shut, meaning the selection highlight isn't visible until you expand it. Making the shortcut collapse-aware — expand on arrival, or skip collapsed groups — requires lifting that state into AppShell. Happy to do that here or as a follow-up, but it restructures state ownership, so I'd rather you called the shape.

Also happy to follow up with

⌥⇧↓ / ⌥⇧↑ for next/previous unread, which is where Slack's mute-skipping behavior properly belongs. Keeping this PR to the plain case.

Checking the keybinding and scope look right before I open the PR.