Channel sections are per-device local storage, so a team never sees the same sidebar

### What happens

Sections organise the sidebar beautifully for the person who creates them, and only for that person. I set up Operations / General / Agents on my machine. The second member joined and saw a flat alphabetical list of 13 channels with no grouping at all.

Sections appear to live in the desktop app's WebView local storage. There is no section concept on the relay, nothing in `buzz channels`, and no config file that could be shipped to another member.

### Expected

In a team workspace, an owner organises the channel list once and everyone sees that structure. This is how Slack, Discord and Teams all behave, and it is what people expect from a sidebar that has headings in it.

### Why it matters

Onboarding is where this bites. A new member's first impression of a workspace is the sidebar, and right now that is an undifferentiated list. Ours mixes per-marketplace execution channels with general discussion and an agent channel, and nothing tells them apart until each person spends a few minutes rebuilding the layout by hand. At six people that is annoying. At thirty it is a real onboarding cost, repeated per person, forever.

It also quietly undermines the channel-per-purpose design the product otherwise encourages. We ended up renaming six channels to `ops-*` purely so alphabetical sorting would group them for everybody. That works, but it is obviously a workaround for a missing feature.

### Suggestion

Store sections on the relay as workspace state, editable by owners and admins, with the current local sections kept as a personal override on top. That preserves today's behaviour for people who like their own arrangement, while giving a team a shared default.

### Environment

- Buzz Desktop 0.11.0, Windows 11
- Block-hosted relay, relay software 0.2.1
- 13 channels, 2 members

Start by tracing where the desktop app stores channel sections in WebView local storage and how the relay exposes workspace state; the issue also notes that `buzz channels` has no section concept. The suggestion is to make shared sections editable by owners and admins while retaining personal overrides. Done means members see the workspace's shared sidebar structure without losing their personal arrangement.

Written by the indexing model from the issue text.