feat: install walkthrough slideshow + push notification dialog

[devin-ai-integration]

Summary

Replaces the terse install toast with a click-through walkthrough dialog (InstallGuideDialog) using 5 edited iOS screenshots (blurred personal info, red tap arrows) in client/portal/public/install-guide/. The dialog shows one screenshot at a time with a caption, progress dots, Back/Next, and Done on the last step.

• Profile page "Install app" always opens the slideshow, on every platform (no more platform-specific toasts / promptInstall() from that button).
• Onboarding InstallPromptHost keeps its toasts, but the iOS toast gains a "Show me how" action that opens the same dialog; closing it settles the onboarding step.
• PushPromptHost no longer shows a toast — the push-permission prompt is now a dialog with "Allow notifications" (loading state while subscribing) and "Not now" buttons; closing dismisses the onboarding step.
• Slideshow card has a constant size across steps: fixed-height image container (h-[min(45vh,24rem)] with object-contain) and a reserved two-line caption (min-h-10), no "Step X of Y" prefix.

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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