## What
The transactions AI consent banner now lets users decline, tells them
the outcome, and stops appearing once they've decided.
- **Dismiss option**: an X button before "Enable AI" permanently hides
the banner without granting consent.
- **Show only until the first decision**: the choice is persisted on
`users.ai_consent_prompt_dismissed_at` (set on both accept and dismiss).
The banner shows only while the user hasn't responded and never
reappears afterwards — even if they later revoke consent from settings.
- **Clear feedback**: accepting or dismissing shows a toast stating
whether AI was enabled and that the choice can be changed anytime in
**Settings > Manage Plan**.
- **Full-width layout on desktop**: the banner content spans the full
available width (button pushed to the right, text on a single line). The
narrow stacked layout is kept for mobile only.
## How
- New `POST ai/consent/dismiss` endpoint
(`AiConsentController::dismiss`).
- `User::dismissAiConsentPrompt()` (idempotent) +
`hasDismissedAiConsentPrompt()`; `store` now also marks the prompt
dismissed on accept.
- Migration adds the nullable `ai_consent_prompt_dismissed_at`
timestamp.
- `TransactionController` passes `aiConsentPromptDismissed` to the page.
## Tests
- `AiConsentTest`: idempotent dismissal, dismissal without consent, and
accept marking the prompt dismissed.
- Full consent + backfill suite and the `create a transaction` browser
test pass; pint / lint / format clean.
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