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' };

backend/src/services/DockerController.ts

@@ -29,6 +29,8 @@ const IGNORE_PORTS = [1900, 53, 22];
 export interface BulkStackInfo {
   status: 'running' | 'exited' | 'unknown';
   mainPort?: number;
+  /** Unix seconds of the oldest running container (approximates stack uptime). */
+  runningSince?: number;
 }
 
 export interface ClassifiedImage {
@@ -660,6 +662,18 @@ class DockerController {
       if (container.State === 'running') {
         result[stackDir].status = 'running';
 
+        // Track the oldest running container's creation time as a proxy for
+        // stack uptime. Docker's listContainers payload exposes Created (unix
+        // seconds) but not StartedAt; for compose stacks the gap is small
+        // enough to treat as uptime without paying for a per-container inspect.
+        const created = typeof container.Created === 'number' ? container.Created : undefined;
+        if (created !== undefined) {
+          const existing = result[stackDir].runningSince;
+          if (existing === undefined || created < existing) {
+            result[stackDir].runningSince = created;
+          }
+        }
+
         // Detect main web port (first running container with a matchable port wins)
         if (result[stackDir].mainPort === undefined && Array.isArray(container.Ports) && container.Ports.length > 0) {
           const ports = container.Ports as { PrivatePort?: number; PublicPort?: number }[];