feat(desktop): improve hot renderer diagnostics

[huntharo]

Summary

• derive hot renderer cpuPercent from cumulative CPU-seconds over wall time so one busy core reports near 100% instead of Electron's machine-normalized 4-6%
• keep Electron's raw normalized reading as electronCpuPercent in samples for debugging/comparison
• add Settings -> General -> Developer mode controls for hot CPU profiling and one-shot smart heap snapshots
• capture bounded heap snapshot batches around hot CPU profiles: start/stop for 2 snapshots, start/mid/stop for 3 snapshots
• auto-clear the smart heap snapshot setting after the configured snapshot budget is consumed, while leaving CPU profiling available
• reconfigure a running renderer profiler when diagnostics settings change, so no app restart or dock-launch env var is required

Test Plan

• /Users/huntharo/.nvm/versions/node/v24.16.0/bin/pnpm test apps/desktop/src/main/__tests__/renderer-hot-cpu-profiler.test.ts apps/desktop/src/main/__tests__/desktop-settings-patch-translator.test.ts apps/desktop/src/main/__tests__/desktop-settings-service.test.ts apps/desktop/src/renderer/src/features/settings/__tests__/settings-screen.test.tsx packages/shared/src/contracts/__tests__/settings.test.ts apps/desktop/src/renderer/src/__tests__/app-shell.test.tsx
• /Users/huntharo/.nvm/versions/node/v24.16.0/bin/pnpm --filter @pwragent/desktop typecheck
• /Users/huntharo/.nvm/versions/node/v24.16.0/bin/pnpm --filter @pwragent/shared typecheck