Auto-select first visible project on cockpit entry

Why: the cockpit rendered the project dropdown with no project selected
and an empty main area — the design's 首屏 default (first visible project
drives the overview) was never implemented; caught by I4 e2e scenario 1
which got past the project list but never saw .cockpit-stats.

Evidence: tsc clean, vitest green, build green (SELF); seven-scenario
rerun to be recorded on the Dev I4 card.

Impact: cockpit entry only.

Refs: rollout I4

src/components/DocumentCockpit.tsx

@@ -70,6 +70,8 @@ export function DocumentCockpit() {
         // rdm.project.list 返回 { projects: [...] }；容忍裸数组形态。
         const rows = Array.isArray(list) ? list : (list?.projects ?? [])
         setProjects(rows)
+        // 设计 §6.2：进入驾驶舱默认选中第一个可见项目，驱动概览首屏。
+        if (rows.length > 0) setPid(prev => prev || rows[0].id)
       })
       .catch(e => { if (!c) setError(e.message) })
     return () => { c = true }