feat: add "Push to Solo" button to TallyView

[mrosseel]

Summary

• Adds a "Push to Solo" button to TallyView.qml next to the existing Cancel / Remove buttons.
• On click, posts the current basket total to backtab's new POST /api/v1/sumup/push endpoint so the bartender doesn't have to type the amount on the Solo by hand.
• Result of the actual card tap is not handled — the bartender confirms it on the Solo's own screen and then continues the existing manual flow (record as deposit, or just take payment outside the tab).

Companion to 0x20/backtab#5.

Behavior notes

• Button is enabled whenever the basket is non-empty.
• A pending entry is added to the log on click; it resolves to "Solo: pushed € X.XX" on success.
• HTTP failures (e.g. SumUp's 422 during the per-reader 60s lockout window) are surfaced in the log as "Solo push failed: 422 ..." so the bartender knows the previous amount is still on the Solo's screen.

Test plan

• With backtab unconfigured for SumUp → click button, log shows "Solo push failed: 503".
• With backtab configured and a paired Solo → add items to basket, click button, verify the Solo wakes up showing the basket total.
• Click twice in quick succession within 60s → second click logs the 422 from SumUp.
• Empty basket → button is disabled.

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