fix(ai-credits): theme token and text contrast fixes

[blueogin]

Description

Addresses the design review from Hadar and Lewis. All changes are scoped to
the ai-credits widget.

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Checklist:

• PR title matches follow: (Feature|Bug|Chore) Task Name
• My code follows the style guidelines of this project
• I have followed all the instructions described in the initial task (check Definitions of Done)
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[Copilot]

Pull request overview

This pull request updates the AI Credits widget UI styling to address design-review feedback by aligning text tones and surface/background tokens with the current theme semantics, improving readability/contrast across views.

Changes:

• Replaced many Text secondary usages with tone="soft" (and a few plain Text) to standardize muted text rendering.
• Updated several containers from $backgroundMuted / $backgroundDark to $backgroundSurface / $background for more consistent surface contrast.
• Updated the How-To content to point to the intended YouTube walkthrough URL.

Reviewed changes

Copilot reviewed 16 out of 16 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/ai-credits-widget/src/components/shared/AddressView.tsx	Adjusts label tone and switches address container background to a surface token.
packages/ai-credits-widget/src/components/setup/SignerKeyPanel.tsx	Tweaks explanatory text tones and related UI copy blocks for improved contrast.
packages/ai-credits-widget/src/components/setup/SetupGuidanceCard.tsx	Updates muted/label text to use tone="soft" for consistent readability.
packages/ai-credits-widget/src/components/setup/SetupFaqView.tsx	Uses tone="soft" for expanded FAQ answers to improve contrast.
packages/ai-credits-widget/src/components/setup/SetupBonusHighlight.tsx	Applies tone="soft" to supporting text and expanded description content.
packages/ai-credits-widget/src/components/setup/HowToUseView.tsx	Updates the walkthrough video URL and applies tone="soft" to helper text.
packages/ai-credits-widget/src/components/setup/AntseedSignerRow.tsx	Applies tone="soft" to caption text beneath the screenshot row.
packages/ai-credits-widget/src/components/manage/SetupSnippet.tsx	Switches snippet container to $backgroundSurface and softens helper text tone.
packages/ai-credits-widget/src/components/manage/CreditsManagementCard.tsx	Uses tone="soft" for stat labels / helper labels.
packages/ai-credits-widget/src/components/manage/BuyerOperatorCard.tsx	Updates surface tokens and softens secondary text to improve contrast.
packages/ai-credits-widget/src/components/history/HistoryTab.tsx	Updates filter/control surfaces and secondary labels to tone="soft".
packages/ai-credits-widget/src/components/buy/OperatorConsentStep.tsx	Uses tone="soft" for helper/bullet text and applies monospace styling via shared style.
packages/ai-credits-widget/src/components/buy/BuyerKeyPanel.tsx	Switches key panels to $backgroundSurface and updates helper/label text tone.
packages/ai-credits-widget/src/components/buy/AmountPicker.tsx	Softens helper/label text across the amount picker UI.
packages/ai-credits-widget/src/components/buy/AiCreditsHero.tsx	Updates label tone for improved contrast.
packages/ai-credits-widget/src/AiCreditsWidget.tsx	Sets the widget root background to $backgroundSurface to better match surface semantics.

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