feat(sidebar): add More unread floating buttons

[wesbillman]

Summary

Adds Slack-style "More unread" floating pill buttons to the sidebar. When unread channels are scrolled out of view above or below the visible area, a directional pill appears showing the count and scrolling to the nearest off-screen unread on click.

What changed

File	Change
desktop/src/features/sidebar/lib/useUnreadOverflow.ts	New hook — IntersectionObserver tracks unread row visibility relative to SidebarContent scroll container; MutationObserver rebinds on collapse/expand. Returns counts + scroll-to-next callbacks.
desktop/src/features/sidebar/ui/MoreUnreadButton.tsx	Extracted pill component matching existing "Jump to latest" style (rounded-full, backdrop-blur, pointer-events layering).
desktop/src/features/sidebar/ui/AppSidebar.tsx	Wraps SidebarContent in a relative container, passes scrollRef, renders top/bottom pills when overflow counts > 0.

Behavior

• Floating pill appears at top/bottom of sidebar when unread channels are scrolled out of view
• Click scrolls to the nearest off-screen unread (smooth, centered)
• Does NOT auto-select the channel — just scrolls it into view
• Collapsed sections excluded from count (can't scroll to what's not in the DOM)
• DMs included via unreadChannelIds
• No wrapping — directional only (matches Slack)

NIT (not addressed — not worth the churn)

• deriveCounts recomputes getRelativeTop from the element rather than caching — keeps logic simpler and avoids stale-position bugs.

Testing

• pnpm --dir desktop typecheck ✅
• pnpm --dir desktop lint ✅
• pnpm --dir desktop test ✅
• Visual QA ✅

[wesbillman]