fix: Recharts dimension warnings + DevTools settings toggle

[aicayzer]

Summary

• Fix `width(-1)/height(-1)` Recharts warnings in `ActivityArea`, `DistBar`, `LineTrend`, and `PaceTrend` by setting `initialDimension={{ width: 1, height: 1 }}` on each `ResponsiveContainer` — overrides the -1 sentinel Recharts uses before ResizeObserver fires
• Add a persistent Enable DevTools toggle in Settings → About → Developer card, wired through the full stack (Config type → config.ts → ipc.ts → api.ts → configStore → Settings UI); DevTools also auto-open on launch when the setting is on

Test plan

• Open app — no `width(-1)/height(-1)` console warnings on any analytics screen
• Settings → About → Developer → toggle Enable DevTools on → DevTools panel opens
• Toggle off → DevTools closes
• Quit and relaunch with toggle on → DevTools auto-open on startup
• Reset app → devTools resets to false
• All quality gates pass: `pnpm typecheck && pnpm lint && pnpm format:check && pnpm test`