Combine plan + credit packs into one onboarding card (APP-5176)

[warp-agent-staging]

Summary

Combines the two AI-purchase options on the account-first "Choose how to start" onboarding slide into a single card, per the APP-5176 design mock. The slide now shows two cards instead of three:

• Card 1 — "Use Warp with AI" (kept green "Recommended" badge): the body copy, a full-width "Subscribe to Warp plan" button, a divider labelled "Or buy AI credits without a subscription", and the existing row of server-priced credit-pack tiles — all in one card.
• Card 2 — "Set up AI later": unchanged.
• The screen also shows the subtitle "To use AI, start with a plan or one-time credit packs." (previously absent).
• Footer (Back / Get Warping) unchanged.

Selection model

Exactly one option is selected at a time across the whole slide:

• Clicking anywhere in the "Use Warp with AI" card selects it (the plan is the default in-card choice); the plan button and pack tiles are nested click targets that take their own selection.
• The "Subscribe to Warp plan" button paints as selected only while the plan is the choice, and a pack tile paints as selected only while that pack is the choice — so the plan and a pack can never look selected at once.
• Choosing "Set up AI later" clears every highlight in the card above.
• "Subscribe to Warp plan" and the pack tiles only select. The upgrade flow / purchase is started by "Get Warping" (or Enter), which acts on the current selection. Packs stay hidden until server pricing arrives; rendering is truncated at MAX_CREDIT_PACKS; the use_warp_with_ai telemetry id is unchanged.
• After "Get Warping" opens a checkout link the footer shows "Waiting for checkout…", but any subsequent selection change (switching to the plan or a different denomination) resets it back to "Get Warping" so another link can be opened — the user is never trapped.

Implementation notes

Confined to crates/onboarding/src/slides/offer_slide.rs (+ its tests) and one small crates/onboarding/src/model.rs helper (reset_pending_checkout). The combined card is one Hoverable using defer_events_to_children so a click the plan button / a pack tile handled wins, while a click on any other part of the card selects it. No warp-server change is needed — credit-pack amounts/prices are server-driven display data.

Deviations from the mock (deliberate — per foreman)

• Badge: kept the existing green "Recommended" badge; the mock's "50% off Fable and Opus 5" pill is intentionally not implemented (out of scope per the requester).
• Card-2 copy: kept the correct wording "…or use another CLI agent."; the mock's "use third your own CLI agent" is an apparent typo and was not adopted.

Verification

• cargo nextest run -p onboarding — 49 passed, including new regression tests: exactly_one_option_is_selected_at_a_time (mutual exclusion + card-body select + set-up-later clears the card above), subscribe_selects_the_plan_without_launching_upgrade, changing_selection_after_checkout_clears_the_pending_state, and changing_denomination_after_checkout_allows_a_new_link (the stuck-checkout fix). Existing packs-hidden / pack-purchase / set-up-later-in-flight / arrow-key / truncation coverage retained.
• cargo clippy -p onboarding --all-targets --tests -- -D warnings — clean; cargo fmt --check — clean.
• Visual proof (video below): drove the running slide with computer use — clicking "Set up AI later" de-highlights the card above; clicking the card body re-highlights it and the plan button; clicking a tile de-highlights the plan button (never both); clicking "Subscribe to Warp plan" opens no page/overlay; "Get Warping" on a pack shows "Waiting for checkout…", and selecting a different tile returns the footer to "Get Warping".

Rework changes

Addressed the requester's Slack feedback on the first pass (no GitHub review threads):

• Selection states were wrong → rebuilt so exactly one option is highlighted; the plan button no longer looks permanently selected, and the plan + a pack can't both look selected.
• Clicking the card didn't select it → the whole "Use Warp with AI" card is now a click target.
• "Subscribe to Warp plan" opened the page → it now only selects; only "Get Warping" starts the upgrade.
• Checkout could get stuck → any selection change now resets "Waiting for checkout…" back to "Get Warping" (added reset_pending_checkout), so a second link can be opened.

Computer-use video recordings

View video recording - Onboarding offer_preview click sequence: Launching the offer_preview onboarding window and performing a sequence of clicks on the two cards, credit tiles, Subscribe button, and Get Warping footer button while observing highlight/label changes.

Computer-use screenshots (3)

View screenshot - The offer_preview app's "Choose how to start" onboarding screen, showing the two cards, credit-pack tiles, divider, and footer buttons on the left half and a decorative screenshot on the right half.

View screenshot - Initial "Choose how to start" screen: Card 1 (Use Warp with AI) has a blue accent border, Subscribe button is accented, Card 2 (Set up AI later) has a plain border.

View screenshot - After clicking "Set up AI later": Card 2 now has the blue accent border while Card 1 and the Subscribe button lost their accent.

CHANGELOG-IMPROVEMENT: Combined the plan and one-time credit-pack options into a single card on the "Choose how to start" onboarding screen.

Originating thread: https://warpdev.slack.com/archives/C0BDQDW8V5E/p1785896644972569

Conversation: https://staging.warp.dev/conversation/d4c70f7e-62e5-414d-980e-257d5ac1bd65
Run: https://oz.staging.warp.dev/runs/019fcfc8-cc3c-798f-b6a6-95df0c78a3ce
Plans:

• APP-5176 ticket

This PR was generated with Oz.