Board (Kanban) view for issues — cross-project state at a glance

## Problem

Buzz is great for working *inside* a project, but there is no way to see the state of *all* projects at once. With six project channels, the only way to know where anything stands is to open each channel and read back through the thread. Project state lives in scrollback, so "what is in flight right now?" takes six clicks and a lot of reading.

## What already exists

The data model is essentially already there. `buzz issues` is implemented (NIP-34, kind:1621) with statuses via `buzz issues status`:

- `open`
- `draft`
- `resolved`
- `closed`

Those four statuses are Kanban columns. The backend exists; the view does not.

## Request

A board view in the Buzz app that renders issues as columns by status.

Minimum useful version:

- Columns: Draft / Open / Resolved / Closed
- Cards show issue title, repo, assignee (`p` tag), and originating channel
- Filterable to a single channel/repo, or aggregated across all of them
- Read-only would already be a big win

Nice to have later:

- Drag a card between columns to publish the corresponding status event (1630-1633)
- Group-by-channel swimlanes so multi-project users get one cross-project glance
- A "last touched" timestamp on each card so stale work is visible

## Why this matters

The cross-project glance is the part that cannot be worked around today. A user can approximate a board by hand-maintaining a markdown table in a channel canvas, but a hand-maintained board goes stale, and a stale board is worse than none because people trust it. Rendering from issue events means the board is always true by construction.

## Context

Raised by a non-technical user evaluating Buzz against other agent workspaces. The specific ask was "something easy to see active state of all projects in one glance rather than going to every thread." The issue tracker being on-relay rather than in GitHub/Linear was a positive for them — this view is what would make it usable without a CLI.