Send button on the campaign send step: make it stand out, remove the clash with Upgrade plan

@Anastasia TurulinaIteration 1

• Send moved into a bar that slides in at the bottom once the header scrolls out of view.
• Upgrade plan in the Schedule block is quieter: a link in a banner instead of a filled button.
• Three variants of the bar in the demo panel.

@Clicked through it, works nicely 👍  Could the upgrade offer show up only when someone tries the feature? Now it’s always on screen.

Iteration 2

• “Finish later” and “Send campaign” sit in the header. When they scroll out of view, a bar with the readiness and the same buttons slides in at the bottom, so there is only ever one send button on screen.
• The button label matches prod: “Send campaign”, and “Schedule campaign” in scheduling mode.
• Resending to non-openers on a plan without the feature: the row stays fully visible with a “Standard plan” badge. A click anywhere on the row opens an upgrade offer under it, and the checkbox doesn’t turn on.
• Not taken: no buttons in the header and the bar always at the bottom. The header buttons stay as on the other pages, and at the bottom they appear on scroll anyway.