Cockpit: group/filter issues by arbitrary label (deferred from v0.5.0)

[robercano]

Blocked by #206

Context

The cockpit groups issues by module:* label only (with a click filter on module headers). During v0.5.0 planning this was considered and explicitly deferred: generalize grouping/filtering to ANY label dimension. Filed here so it's ready for triage into the release after v0.5.0.

Proposal

1. On the SPA, a group-by selector (module:, priority:, from:*, plain labels) and a free filter (include/exclude label chips) applied to the issue list and, where sensible, the release board.
2. Pure client-side over Cockpit v2 foundation: /api/state JSON snapshot + zero-dep client-rendered SPA shell #206's state JSON — no new server work expected.
3. Persist the chosen grouping/filters client-side (localStorage, same pattern as the theme toggle — inert under file://).

Acceptance criteria

• Group by any label prefix or exact label; filters compose (e.g. priority:high + exclude needs-human).
• Selection survives reload; default remains today's module grouping.
• No extra server round-trips; works against fixtures.

Notes

• Not milestoned on purpose — backlog for the release after v0.5.0, per the planning session decision.