feat: auto-open install walkthrough on mobile browsers instead of toast

[devin-ai-integration]

Summary

Removes all the install onboarding toasts. InstallPromptHost now opens the InstallGuideDialog walkthrough automatically for any logged-in user on a phone browser (platform === "ios" | "android") that isn't running standalone — on every visit, not just the first (the dialog's open state is derived, not gated by the install-prompted localStorage flag):

const guideOpen = !!user && !installed && isMobile && !closed; // closed = dismissed this visit

Closing the dialog still dismiss()es (settles the onboarding step so push/schedule-tip aren't blocked). On desktop there is nothing to install, so the step is settled immediately via an effect. The native beforeinstallprompt Install action is gone along with the toasts; promptInstall/canPromptNatively remain available in useInstallPrompt.

Also drops the min-h-72/mt-auto from the "Get Notified" push dialog so there is no empty gap between the description and the buttons.

Link to Devin session: https://app.devin.ai/sessions/3819fb8e090243e3be94e42b6634f587
Open in Devin Desktop: https://app.devin.ai/desktop/session/3819fb8e090243e3be94e42b6634f587?variant=devin
Requested by: @balebbae

[devin-ai-integration]

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