feat(desktop): nest channel sections one level deep

[s-kikeriki]

What this adds

Sidebar sections are a flat list today. This lets a section hold other sections one
level down, each with their own channels, order and icon.

Closest existing issue: #5738 (the original custom sections request). No open issue or
PR asks for nesting specifically — I searched issues and PRs for "subsection", "nested
section" and sidebar section titles before starting. Happy to convert this to an issue
first if you would rather agree the direction before reviewing code.

How it works

• A section carries a parent register in the existing kind 30078 lane, so nesting
  syncs across devices and survives a reinstall like the rest of the sidebar state.
  No new event kind.
• The projection caps the tree at two levels and treats a parent that is missing,
  deleted, self-referential or already nested as "no parent". A corrupt document, or
  one written by a newer client, degrades to a flat list rather than hiding channels.
• The legacy payload older clients read gains a parentId field. Clients that ignore
  it keep rendering every section flat, so this is additive on the wire.

UI

• "New subsection..." on a top-level section, "Move out of subsection" on a nested
  one, both in the ⋮ menu and the header context menu.
• Subsections indent under their parent, collapse with it, and roll their unread count
  and "Mark all as read" up into it.
• "Move to section" lists subsections indented, with the parent spelled out in the
  accessible name.
• Only top-level sections take part in drag reordering; a subsection reorders within
  its siblings.

Trade-offs

• One level only. Two levels covers the grouping people ask for and keeps the
  projection rules small enough to reason about; arbitrary depth would need cycle
  handling on every read path.
• Mobile renders the subsections flat for now. The payload carries the parent, so
  mobile can pick it up separately.

How to test manually

1. Right-click a section header, "New subsection...", name it.
2. Right-click a channel, "Move to section", pick the subsection.
3. Collapse the parent and confirm the subsection and its channels collapse with it.
4. Reload. The nesting is still there.
5. "Move out of subsection" on the nested section returns it to the top level.

Tests

• Projection rules: depth cap, cycles, orphan promotion, channel assignment.
• Hook guards.
• A Playwright spec that creates a subsection, moves a channel into it and reloads.

Deferred

• Mobile rendering of the nesting.
• Sharing a section layout across a team (Sidebar channel sections are per-user only — no way to share a section layout across a team #5469, Channel sections are per-device local storage, so a team never sees the same sidebar #6471) is unaffected by this change.

Screenshot

"Clients" holds #random, with "Kalte Herberge" and "Schweizerhof" nested under it,
each with their own channels.

[github-actions]

🔐 Codex Security Review

Status: review required for the current range.

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