fix(popup): report split-tunneling state on the collapsed row

[dantraynor]

What

The Split tunneling row rendered a toggle switch that only opened and closed the editor, with its state in a module-level variable that resets whenever the popup document is recreated — so reopening the popup showed an off-looking switch styled exactly like the Shields Up and MagicDNS preference toggles while the saved rules were still in force. It is now a disclosure row modelled on Exit Node: a summary value (Off, Bypass · 2 domains, Only · 1 domain) plus a chevron that rotates when expanded, with aria-expanded/aria-controls, refreshed both optimistically on save and from state in updateConnected. Only with an empty domain list is deliberately not reported as Off, since that combination routes nothing through the exit node.

Why

Closes #116

How to Test

• Connect, pick an exit node, expand Split tunneling, add a domain and hit Save rules
• Close and reopen the popup — the collapsed row reports Bypass · 1 domain instead of an off-looking switch
• Clear the domains in Bypass mode and confirm the row reads Off; switch to Only with an empty list and confirm it reads Only · no domains
• Click the row to expand and collapse; the chevron rotates and the editor follows

Checklist

• Tested in Chrome
• Tested in Firefox
• Updated README if needed

Verification: pnpm typecheck clean, 571 unit tests pass (4 new in connected.test.ts), and the full e2e suite passes in Chrome (26 cases) and Firefox (20 cases). The split-tunneling e2e scenario now reopens the popup after saving to assert the row still reports the saved rules — the exact regression reported in the issue.