Fix Exo window resume speed and duplicate sync initialization

[mickn]

What this fixes

Closing Exo's last macOS window could destroy the renderer, so Dock activation recreated the app from scratch, losing navigation state, and re-ran account initialization—all resulting in having to re-select the original mailbox, and having to wait 10+ seconds before the app was usable again. Repeated initialization could also replace account records without clearing old interval handles.

Changes

• Hide the last macOS window on normal close and reuse it on activation; intentional app quit still closes normally.
• Coalesce concurrent sync:init calls and reuse healthy main-process Gmail clients.
• Clear replaced sync intervals and coalesce overlapping account syncs.
• Persist and validate a compact navigation snapshot for genuine renderer loss or restart.
• Add lifecycle, navigation, and Electron resume regression coverage plus diagnostic counters.

Verification

• npm run build
• Node and renderer TypeScript checks
• Focused lifecycle/navigation tests
• 1,461 unit cases green (localhost-binding tests rerun with required permission)
• Native Electron E2E confirms the same webContents survives close → activation

This branch is rebased onto ankitvgupta/exo main at 5c22a58.