perf(gui): unblock dashboard/models controls and stabilize providers rail

[Wibias]

Summary

• Dashboard: load settings/sidecar/shadow via a dedicated poll (not gated on slow health/usage); session cache for last-known; aria-busy on slow stats only
• Models: fetch shadow settings on mount so switch/dropdown work while catalog still loads
• Providers: keep rail order config-stable (no alpha→ready resort flash); prefetch catalog/usage; parallel oauth/accounts

Before/after notes: devlog/gui-load-perf-hot-before-after.md

Test plan

• Dashboard: Shadow Call Intercept + sidecar/vision usable before tokens/uptime finish
• Models: shadow controls interactive during catalog load
• Providers: rail does not jump from alpha to ready/need-setup after discovery
• Add Provider catalog feels snappier on open
• gui lint + dashboard-contracts / provider-workspace-data tests

Part of GUI loading split (PERF-1). UI drafts: #607 #630 #629.

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[Wibias]

Superseded by a single combined GUI UX/perf PR from the local local/gui-ux-perf-bundle worktree (chrome + storage + dense workspaces + hot-path + page-data + later polish).

Closing this split PR to avoid duplicate review; the replacement PR will land against dev shortly.