Surface PhoneNotResponding instead of swallowing it (cryptic UNKNOWN sends)

[MaxGhenis]

Problem

internal/client/events.go receives libgm's PhoneNotResponding / PhoneRespondingAgain events but only logs them (line ~90). The user never learns the phone is unreachable — outgoing sends just fail with a bare UNKNOWN, and inbound silently stops.

Ask

• Track a phoneResponding bool on the App (set false on PhoneNotResponding, true on PhoneRespondingAgain / ClientReady / a successful inbound message), exposed in GoogleStatusSnapshot (e.g. phone_responding).
• When a send fails while phone_responding == false, tailor the error ("your phone isn't responding to OpenMessage right now — make sure it's on and online") instead of the generic re-pair guidance from Detect a stuck Google Messages session and offer re-pair #42.
• Optionally show a subtle "phone unreachable" indicator in the UI.

Complements #42 (which handles the unlinked-device case). This handles the phone-temporarily-offline case, which #42's repeated-failure heuristic would otherwise eventually misattribute to a stuck session.

[tpmeredith]

I’m taking this one and will link a draft PR once the fix is tested.