Redesign settings dialog: modern macOS toolbar-style layout with proper form alignment

[Copilot]

The settings dialog used an outdated NSTabView card UI with left-aligned labels, inconsistent VFL spacing, and a fixed 440×300 window — far from current macOS conventions as seen in Safari/Mail Settings.

Navigation

• Replaced bare NSTabView with NSTabViewController (NSTabViewControllerStyleToolbar) — icon toolbar across the top, content below, matching Safari/Mail Settings on macOS 13+
• Window title changed to "Settings"; window auto-resizes per pane via preferredContentSize
• SF Symbol icons per tab (gearshape, paintbrush, doc.text) with @available(macOS 11.0, *) guard and system-image fallbacks

Form layout

• Replaced all manual VFL constraints with NSGridView: right-aligned label column (NSGridCellPlacementTrailing, fixed 140 pt) + left-aligned control column — all controls start at the same x-position across every pane
• Checkboxes placed in the control column with NSGridCell.emptyContentView in the label column (they are self-labelling; no duplicate label)
• Numeric field + stepper + unit label composed via a reusable NSStackView helper (stepperRowWithField:stepper:unit:)

Spacing

Named constants codify HIG-standard values throughout:

static const CGFloat kEdgeInset        = 20.0; // pane margins
static const CGFloat kLabelColumnWidth = 140.0; // right-aligned label column
static const CGFloat kColumnGap        =  8.0;  // label → control gap
static const CGFloat kRowSpacing       =  8.0;  // between form rows

All action methods are unchanged.

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