feat: detached-runs monitor page (T3 parity)

New Runs sidebar destination listing detached invocations (detach:true)
with phase, current step, project and a Cancel button while
pending/running. Workspace-scoped like Audit/Approvals, polls every 2s.
Detached runs are opt-in (detached.enabled) — the empty state explains
how to enable them. Inline help doc DE+EN. DetachedRun model +
listDetachedRuns/cancelDetachedRun in HubService, backed by the SDK's
listInvocations()/cancelInvocation(). flutter analyze clean; 29 tests
green (sidebar Y-stability updated for the new destination, model
mapping unit-tested).

Screenshot verification (light+dark) deferred — shared desktop in use.

Signed-off-by: flemming-it <stefan.a.flemming@googlemail.com>

assets/docs/runs.md

@@ -0,0 +1,45 @@
+# Runs
+
+The Runs page monitors **detached invocations** — flows submitted
+to run in the background rather than being waited on. Each row shows
+the flow, its current phase, the step it is on, and the project it
+was stamped with.
+
+## What a detached run is
+
+Most flows run *inline*: you start them and wait for the result. A
+**detached** run is handed to the hub and returns an id immediately;
+the flow keeps running in the background. You come back later to
+read its result, follow it live, or cancel it.
+
+Detached invocations are **opt-in**. They only work when the
+operator enabled them in the hub config:
+
+```yaml
+detached:
+  enabled: true
+```
+
+Until then this page is empty — that is the normal state.
+
+## Phases
+
+- **Pending** — accepted, not yet executing.
+- **Running** — a step is executing (the row names it).
+- **Succeeded** — finished; the result is retained under the hub's
+  size/count/TTL limits.
+- **Failed** — finished with an error (shown on the row).
+- **Cancelled** — stopped by an operator.
+
+## Cancelling
+
+A pending or running detached run carries a **Cancel** button.
+Cancelling signals the run to stop; a run that already finished
+cannot be cancelled (Studio says so rather than pretending).
+
+## Workspace scope
+
+The page respects the active workspace: with a project selected it
+shows only that project's detached runs. Detached results are held
+in memory for the current hub process — they are not retained across
+a hub restart.