fix(channels): flatten the list and make the panes swipeable

[puemos]

Problem

Design follow-ups on the Channels sidebar slider in #3814.

The list read as a tree when it isn't one: "Starred" and "Channels" carried a #
(so they scanned as channels themselves) and the rows under them were indented,
as if a channel belonged to its heading. The ⌘1-9 hints sat at full strength
and pulled the eye off the channel names they annotate. And the back row was the
only way out of a channel — no gesture.

Changes

• Flat list. The group headings lose the # and the indent under them, so a
  channel sits at its heading's level and the glyph (#, or a lock once channels
  can be private) belongs to the row that has it. The heading's icon well rests
  empty and fills with a chevron on hover/focus.
• Dimmer shortcut hints. ⌘1-9 at 50% opacity, still hidden on hover.
• Swipe between the panes. A two-finger horizontal swipe goes out to the
  channel list and back into the channel you're still scoped to
  (useChannelPaneSwipe). A trackpad swipe arrives as a horizontal wheel and the
  sidebar has nothing to scroll sideways, so the panes claim those, accumulate
  the deltas across the gesture, and lock until the wheel goes quiet — otherwise
  the momentum tail swipes straight back.

All three are under the channels layout flag; the alpha's channel tree renders
exactly as it does today, which the tests pin.

Based on posthog-code/channels-stack-3-spaces-ui so the changes land in #3814
— the signed-commit tooling has that branch locked as this session's base and
refuses to commit to it directly.

How did you test this?

• pnpm --filter @posthog/ui test — 249 files / 2192 tests pass, including new
  coverage for the swipe (back, forward, delta accumulation, vertical scroll
  ignored, momentum lock) and for the flat-vs-indented grouping on each side of
  the flag.
• pnpm --filter @posthog/ui typecheck and Biome clean.
• Not run in the app — the swipe feel and the animation are unverified visually.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit b8c3146.}

[posthog]

👋 Visual changes detected for this PR.

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