Replace inline pink color with reusable .btn-prompting CSS class on prompting agent pages

[Copilot]

Prompting-strategy agent pages (Agent17–23) were using a hard-coded #e83e8c inline style for their primary Submit button, bypassing the project's CSS variable system.

Changes

• app.css — Added .btn-prompting, .btn-prompting:hover, and .btn-prompting:disabled rules in the Buttons section, backed by var(--color-cat-prompting):

  .btn-prompting {
      background-color: var(--color-cat-prompting);
      border-color: #c72d7c;
      color: var(--color-white);
      ...
  }

• Agent17–23.razor — Replaced inline style="background-color:#e83e8c;color:white;" with class="btn btn-prompting" on each Submit button.

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