cmd/tsconnect: use empty string as the default state store key

Makes the Wasm client more similar to the others, and allows the default
profile to be correctly picked up when restarting the client in dev
mode (where we persist the state in sessionStorage).

Also update README to reflect that Go wasm changes can be picked up
with just a reload (as of #5383)

Signed-off-by: Mihai Parparita <mihai@tailscale.com>
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