feat: opt-in auto-connect on start preference

[dantraynor]

Summary

Adds an opt-in Auto-connect on start toggle to the connected view's quick settings (off by default). When enabled, the extension sends up once per browser session if the first status after init reports Stopped or NoState. NeedsLogin / NeedsMachineAuth / Starting / Running / InUseOtherUser are skipped.

An explicit manual disconnect within the same browser session is respected: chrome.storage.session tracks an "already handled" flag that survives MV3 service-worker restarts but clears on browser restart, so cycling the SW after a manual down won't re-up the node. There's a defensive fallback if storage.session is unavailable — the feature still works, just without the explicit-disconnect guarantee.

Design choices (from the issue's open questions):

• Global — single autoConnectOnStart boolean in chrome.storage.local, matches the existing lastExitNodeID pattern.
• Quick settings — toggle lives next to Shields Up / MagicDNS in the connected view.
• Respect explicit disconnect within a session — see above.

docs/FEATURE_PARITY.md line 35 flipped to Yes with a note describing the new behavior.

Closes #73

Test plan

• pnpm typecheck — clean across both packages
• pnpm test — 296/296 pass (27 new tests; covers the auto-up matrix, session-flag interaction, manual-disconnect path, and the missing-API fallback)
• pnpm build — Chrome + Firefox MV3 bundles build cleanly
• Manual: enable toggle while connected, restart browser, confirm Tailscale comes up automatically
• Manual: with toggle on, click disconnect, close popup, wait for SW to idle (>30s), reopen popup — should remain disconnected
• Manual: with toggle on, full browser restart after manual disconnect — should auto-connect again
• Manual: with toggle off, restart browser — no auto-connect