refactor(frontend): extract shared render/format helpers into util.js (#1451)

The tabs.js split's render-heavy domains (schedules, system, containers,
questions/approvals) all share a handful of pure helpers that lived in
the tabs.js IIFE: paintAtomic (atomic-swap render) and the fmtAgo /
fmtElapsed / fmtDuration / truncate formatters (used 10/15/4/6/6 times
across the file). Pull them into a new dashboard-internal util.js so the
upcoming per-tab modules can import them instead of depending on the
entry's closure — the helper analogue of the state.js roster keystone.

They stay out of the cross-page common.js (their phrasing is
dashboard-specific) but are now a shared dashboard module. All five are
pure, so this is behaviour-preserving code motion; esbuild inlines util.js
into the tabs.js bundle. Build green.

frontend/packages/dashboard/src/common.js

@@ -46,10 +46,11 @@ export const form = (action, btnClass, btnLabel, confirmMsg, extra = {}, opts =
   return f;
 };
 
-// `truncate`, `fmtAgo`, `fmtElapsed`, `fmtDuration` stay in tabs.js
-// for now — each has display-specific phrasing ("X running", "X ago")
-// tied to its caller, so they don't generalise cleanly. We can lift
-// them when a second consumer needs the same shape.
+// `truncate`, `fmtAgo`, `fmtElapsed`, `fmtDuration` + the `paintAtomic`
+// render helper live in the dashboard-internal `./util.js`, not here —
+// their phrasing ("X running", "X ago") is dashboard-specific, so they
+// stay out of the cross-page `common.js` but are shared across the
+// dashboard's own tab modules.
 
 // ─── shared-worker SSE pipe ─────────────────────────────────────────────
 // Returns an EventSource-shaped facade backed by a SharedWorker that