Simplify Dashboard Uptime onto the shared 7-second hero poll

ROADMAP.md

@@ -202,3 +202,6 @@ Roughly in priority order:
 `v0.16.37` combines the documentation catch-up for the hero panel arc (v0.16.29-36: README feature bullets and the environment-variable table, plus new "Live resource panel" and "Version" sections in the in-app manual's Administration System article, and an updated Dashboard "Runtime & System" article) with a real fix found while reviewing the Dashboard's hero panel live: the Throughput chip next to it (requests/min) was still only updating on the old 30-second `refreshDashboard()` timer, not the hero's 7-second poll, even though `/api/system/health` already computes that exact number on every call and the Dashboard was just discarding it. `refreshDashboardHero()` now also updates the Throughput chip from that same response, so it refreshes on the same cadence as the rest of the hero instead of lagging behind it by up to 23 seconds.
 
 `v0.16.38` fixes the Dashboard hero's Uptime tile reliably showing "0m" right after a page load or refresh, before slowly counting up from there rather than showing the real elapsed time immediately. Root cause: a 1-second ticker (`setInterval(() => updateDashboardUptime(), 1000)`) has always run independently of the real data fetch, calling `updateDashboardUptime()` with no argument once a second while the Dashboard is visible. The function's old anchor logic (`window.__dashboardStartedAt || (window.__dashboardStartedAt = ...)`) treated a bare, argument-less call as "anchor starts now" (zero elapsed), and because that anchor was set-once, a later call carrying the real `uptimeSeconds` from `/api/dashboard` was then a no-op -- the wrong zero-based anchor had already won the race, almost every time, since the ticker fires every second and the dashboard fetch takes at least one network round trip. `updateDashboardUptime()` now only ever sets the anchor from a real, finite `seconds` value, and does so every time real data arrives rather than once -- so it can't be raced by the bare ticker call (which now just re-renders using whatever anchor already exists, or does nothing until one does), and it also self-corrects if the container genuinely restarts while the page stays open, instead of drifting forever from a stale first anchor. Separately, confirmed by inspection (not a bug, but worth documenting): the Administration System tab's Version panel does *not* tick client-side the way the Dashboard's hero does -- its Uptime is a static string recomputed only when the page's shared dashboard data refetches (on load, or every 30 seconds while the Dashboard view specifically is the active one), so it can go visibly stale while sitting on the Administration tab. Left as-is for now since it's presented as build/version metadata rather than a live stat, but flagged in case a ticking version is wanted there too.
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+`v0.16.39` retires the Dashboard Uptime tile's separate 1-second client-side ticker, the same mechanism behind the v0.16.38 "resets to 0m on page load" bug, in favor of treating Uptime as just another field on the shared 7-second `/api/system/health` poll that already drives CPU, memory, swap, disk, and network on both the Dashboard and the Administration > System tab's hero panel. Revisiting the ticker after fixing its race condition, it turned out to be solving a precision problem the display doesn't actually have: `formatDuration()` only ever renders minute-level granularity ("2h 59m"), never seconds, so a per-second tick never changed what was on screen between one 7-second poll and the next. `systemHealthSnapshot()` now includes `uptimeSeconds` (from `process.uptime()`, the same source `dashboardSnapshot()` already used), and `renderHeroPanel()`'s former `includeThroughput` boolean became a `sixthSlot` option ("throughput" for the System tab, which has no other requests/min display, or "uptime" for the Dashboard, which already shows Throughput in its own chip) so the sixth hero slot can be either stat without a special case. The client ticker, its anchor state, and the whole race-condition class it enabled are gone: one poll, one code path, six stats, no anchor to get out of sync.