fix: pin onboarding action buttons on small screens

[Copilot]

On small screens, primary action buttons (Next, Login, Skip, Finish) in the onboarding flow were cut off because they lived at the bottom of content columns that could overflow without scrolling.

Approach

Introduced an optional footer parameter on _StepWrapper that renders outside and below the scrollable content area, always visible regardless of screen height.

class _StepWrapper extends StatelessWidget {
  final Widget content;  // now wrapped in Expanded — scrollable
  final Widget? footer;  // pinned below, always visible
  ...
  // In build():
  Expanded(child: content),
  if (footer != null) ...[
    const SizedBox(height: 12),
    footer!,
  ],
}

Per-step changes

• Language — language list scrollable; Next button → footer
• Theme/Appearance — removed LayoutBuilder/ConstrainedBox wrapper; settings list scrollable; Next button → footer
• Login (search mode) — school list retains Expanded; manual entry + demo mode buttons → footer
• Login (manual/filled mode) — form fields only in SingleChildScrollView; login + demo mode buttons → footer
• Gemini — info box + API key field scrollable; Skip/Next row → footer
• Tutorial/Finish — feature list scrollable; Finish button → footer

[Copilot]

Pull request overview

This PR fixes onboarding flow action buttons being cut off on small screens by restructuring step layouts so the main content can scroll while primary actions remain pinned and always visible.

Changes:

• Added an optional footer to _StepWrapper and rendered it below the scrollable content.
• Updated onboarding steps (Language, Appearance, Login, Gemini, Tutorial/Finish) to move primary action buttons into the pinned footer area.
• Wrapped step content in SingleChildScrollView where appropriate to prevent overflow on short viewports.

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