Lazy-load route panels for code splitting

[epatey]

Caution

It's fine to defer merging this branch for a few days past Wednesday to be conservative.

It is a relatively safe change. The changes are all at the app router layer making ALL routes lazy. That is, there's no manually coded deferred imports within any of the panels. It's all at the panel level itself.

Summary

• Replace static imports of route-level panel components (ScansPanel, ScanPanel, TranscriptsPanel, TranscriptPanel, ProjectPanel, ValidationPanel, ScannerResultPanel, RunScanPanel) with React.lazy + dynamic import()
• Wrap <Outlet /> in a <Suspense> boundary with <LoadingBar> as the fallback
• Remove unused element property from ActivityConfig and its static panel imports in activities.tsx

Motivation

The initial JS bundle includes all route panels even though only one is rendered at a time. Lazy loading splits each panel into its own chunk, reducing the initial payload and improving time-to-interactive — especially for the common case where users land on a single route.

Impact

Initial download size per top level route (measured from browser network tab):

Route	Before (kB)	After (kB)	Saved (kB)	Change
Transcripts	9,594	1,520	8,074	-84%
Scans	9,594	1,911	7,683	-80%
Project	9,594	1,551	8,043	-84%

The biggest win comes from no longer eagerly loading MathJax (tex-svg-full, xypic) on routes that don't render transcripts.