App detail: merge Instances into Summary, scaling UX, live instance telemetry

[nabramovitz]

Summary

Consolidates the CF app-detail Instances experience into the Summary tab and restores/clarifies instance scaling + telemetry — signal-native throughout. The Prometheus Metrics tab is untouched.

• Merge & route: fold the Instances tab content into Summary; drop the standalone tab; legacy …/instances redirects to summary (deep-links repointed).
• Scaling UX: Summary scale card gets explicit Running/Desired labels, in-place controls, scale-down floored at 0 (no negative requests), scale-to-zero confirmation preserved, label/input associated.
• Usage gauge: restore the per-instance percentage (148 / 256 MB (58%)).
• Instances accordion: gates live polling — expand = live (5s, adjustable 5s/10s/30s); collapse = frozen last values + timestamp; proportion-bar header (amber when running < desired).
• Live trend charts: CPU/Mem/Disk sampled from the existing app-stats poll into a capped in-memory ring buffer (no Prometheus needed); humanized byte axes.
• Legend behavior: per-metric instance hide that persists across the live refresh (enforced via a chart plugin, since Chart.js drops dataset.hidden on update), plus a momentary All metrics overlay that applies a selection across all three charts (AND-merge to reconcile differences) and auto-restores per-metric state on deactivate.

Test plan

• Frontend unit/component tests (vitest) green locally; full make check gate green prior to the legend follow-ups.
• CI gate (lint + frontend vitest + backend go).
• Manual: Summary shows collapsed Instances accordion (proportion bar + timestamp); expand → live table + 3 charts build; scale 0↔1; change sample interval; per-metric legend hide survives a poll; All-metrics overlay applies/reverts; …/instances redirects to Summary; Metrics tab still works where a metrics endpoint is connected.

[norman-abramovitz]

LGTM