feat(web): done dot on sessions that finished while unviewed

A session that stops running while it is not the selected session arms a
green 'done' reminder dot on its sidebar row, so the operator notices a
finished background session and returns to it; opening the session clears
the dot, and a re-run re-arms it on completion.

SessionManager owns the reminder set (a sibling of the waiting-approval
bit): a running->idle edge of a non-selected session arms it, select()
consumes it, removal prunes it, and it survives connection generations.
The bit rides SessionListEntry/SessionSummary into the workspace browser
rows, which render the existing StateDot done state (running keeps the
spinner) and label the hover card '已完成/Completed'.

packages/client/runtime/tests/lineage.spec.ts

@@ -52,4 +52,11 @@ describe('flattenLineage', () => {
       warnSpy.mockRestore()
     }
   })
+
+  it('projects the completion-reminder set into rows (absent = false)', () => {
+    const out = flattenLineage([s('a', 10), s('b', 20)], undefined, new Set(['b' as SessionId]))
+    expect(out.find(e => e.sessionId === 'a')?.completed).toBe(false)
+    expect(out.find(e => e.sessionId === 'b')?.completed).toBe(true)
+    expect(flattenLineage([s('a', 10)])[0]?.completed).toBe(false)
+  })
 })