Double-click a tile to zoom it

[evandhoffman]

Summary

• Double-click any card or dial to zoom it. Escape, Done, or another double-click closes it.
• The zoom is a sheet over the panel, not a second window and not an in-place expansion. It is temporary in the way the other two are not: Escape already means dismiss, and nothing is left over to find later. A second window would need its own toolbar, its own history window and a relaunch story; expanding in place leaves the panel in a state that reads as a bug if you walk away.
• Zoom is a mode the panel is in, not a property a tile carries. One @State value on DashboardView holds it, so opening a second tile closes the first by construction. It stays out of PanelArrangement — a zoom must not survive a relaunch or resize anything stored, and the tile comes back the size it was.
• ZoomLayout (Sources/MonitorUI/ZoomedCard.swift) turns the measured panel size into the sheet's: 86% × 82%, bounded to 520×360 and 1400×900. The margin of panel left showing is what says "temporary"; the ceiling is where a zoom stops being reading a chart and starts being stretching one.
• Sampling does not stop while the zoom is open. The buffer is shared, so the panel underneath keeps filling and comes back without a gap.
• DashboardView.chartCard extracted so the grid and the zoom build the same card at different plot heights, and gaugeTile now takes its size so the caption scales with the dial.

Closes #29

Test plan

• swift test — 185 tests in 21 suites pass, 10 of them new in Tests/MonitorUITests/ZoomLayoutTests.swift
• swiftformat Sources Tests --lint --cache ignore clean
• swift run monitor: double-click a chart, then a gauge — the second closes the first
• Escape closes the zoom; so does Done; so does a second double-click
• The three gestures on a tile do not fight: drag to reorder still works, right-click still opens Copy Image / Copy Data, and neither fires on a double-click
• The panel behind the zoom has no gap in its history when the zoom closes

Out of scope

A zoomed chart has room for more than a bigger version of the same picture — more x-axis labels, and a longer history window than the panel's. Worth a follow-up; this PR draws the same card at a larger size.