Fall back to opening the goal popover below, tracked via a scroll-safe coordinate space

[gsbernstein]

What changed

Re-implemented the "prefer above, fall back to below when there isn't room" behavior, this time using a technique that actually tracks the button's position correctly while scrolling.

Why the previous attempt failed

The earlier version used the common GeometryReader + .background + .global + PreferenceKey pattern to read the goal button's on-screen position. That pattern is well known to not reliably update during interactive ScrollView scrolling — .global frame values tend to only get recomputed when something else forces a re-render of that subtree, not simply because the user dragged the scroll view. In practice this meant the "space available above the button" was based on a stale position (most likely wherever the button was when the screen first appeared), so the fallback logic was comparing against the wrong numbers and would pick the wrong edge almost regardless of actual current scroll position.

The fix

• ContentView's ScrollView now declares a named coordinate space (Constants.mainScrollCoordinateSpaceName) and publishes its own viewport size via the new scrollViewportSize environment value, measured with onGeometryChange (the ScrollView's own frame doesn't move as its content scrolls, so this value is stable/correct).
• SleepBankCard's goal button reads its frame in: .named(mainScrollCoordinateSpaceName) via onGeometryChange, which — unlike .global, per the documented ScrollView "frame layer" technique — does update correctly and continuously as the user scrolls, since it's relative to the ScrollView's own fixed viewport rather than the whole screen.
• goalPopoverArrowEdge compares the button's live position within that viewport against an estimated popover height to decide whether to open above (arrowEdge: .bottom, preferred) or below (arrowEdge: .top, fallback) — now driven by continuously-correct data instead of stale/guessed data.

Files changed

• Bedtime/Bedtime/Constants.swift: new mainScrollCoordinateSpaceName.
• Bedtime/Bedtime/ContentView.swift: declares the coordinate space on the main ScrollView and publishes its viewport size via environment.
• Bedtime/Bedtime/Views/SleepBankCard.swift: reads the goal button's frame in that coordinate space and computes the arrow edge from it; preview updated to set up the same coordinate space/environment so it stays interactive.

Verification

This is a native iOS/SwiftUI change. Per the repo's AGENTS.md, this Cloud Agent environment runs Linux and cannot build/run Xcode or the iOS Simulator, so this has not been visually verified. Please confirm on device/Xcode that:

• It still opens above in the common case.
• Scrolling the card so the button is near the top of the screen and then tapping "Goal" now correctly falls back to opening below, and that this is correctly re-evaluated as you scroll (not just based on the initial position).