feat(dashboard): status masthead, unified gauges, stack health sparklines (#676)

* feat(dashboard): status masthead, unified gauges, stack health sparklines

Rework the home dashboard around a single cyan-railed status masthead
that carries the health state word, node meta, and reasons inline. The
resource block collapses into one strip with a CPU hero sparkline,
memory and disk gauge bars, and a network tile whose sparkline is built
from per-container byte-counter deltas. Stack health becomes an
8-column mono table with row tinting, an uptime column sourced from
the oldest running container's creation time, and a per-stack 10-minute
CPU sparkline. Historical charts pick up a cyan gradient and an amber
peak marker. A shared Sparkline primitive backs all of the above.

* docs(dashboard): refresh screenshot for phase B layout

* fix(dashboard): anchor sparkline bucketing to latest metric timestamp

The cpuHistory, netHistory, and cpuPeakLabel memos called Date.now()
inside useMemo, which violates react-hooks/purity: the rule fires
because re-renders can produce different bucket boundaries from the
same inputs. Derive a historyEndAt anchor from the newest metric
sample in the polled series and thread it through to ResourceGauges.

backend/src/index.ts

@@ -4376,7 +4376,7 @@ app.get('/api/stacks/statuses', async (req: Request, res: Response) => {
         const dockerController = DockerController.getInstance(req.nodeId);
         const bulkInfo = await dockerController.getBulkStackStatuses(stackNames);
         // Map back to filenames to match frontend expectations
-        const data: Record<string, { status: 'running' | 'exited' | 'unknown'; mainPort?: number }> = {};
+        const data: Record<string, { status: 'running' | 'exited' | 'unknown'; mainPort?: number; runningSince?: number }> = {};
         for (const stack of stacks) {
           const name = stack.replace(/\.(yml|yaml)$/, '');
           data[stack] = bulkInfo[name] ?? { status: 'unknown' };