Build Preview mode rendering

The card body's resting state: swift-markdown (pinned 0.8.0, smart
typography off — Preview renders the bytes on disk) parsed into a pure
BodyMarkup model with UTF-8 source offsets, rendered on one hosted
TextKit 1 NSTextView — chosen because find-in-text is NSTextFinder,
checkbox clicks reuse AppKit character hit-testing, links are .link
attributes, and NSTextTable's automatic layout is exactly the
columns-sized-to-contents rule. The GFM subset renders per 05; HTML
stays verbatim code-styled text; relative images resolve against the
card folder while remote URLs are never fetched, drawing a quiet chip
instead. Task checkboxes are live: a click flips exactly one byte
through a fresh-read, refuse-uneditable, stamp, atomic-replace write —
the app's only offset-addressed write, so a moved target refuses as
staleTarget and what the user saw decides the direction, netting one
toggle on a double-click. Empty bodies open in Edit per CardBodyMode's
opening rule, applied once; the Edit surface itself stays an honest
read-only stub until its card. FindCommand prefers the card body's
find over board search when a card window is focused.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/UI/Card/CardWindowMetrics.swift

@@ -79,6 +79,32 @@ enum CardWindowMetrics {
         columnWidth(characters: bodyMinimumCharacters, bodyPointSize: bodyPointSize)
     }
 
+    // MARK: - The rendered body
+
+    /// One step of structural indent in Preview — a list level, a quote level. One and a half ems,
+    /// which is wide enough for a bullet plus its space and narrow enough that four nested levels
+    /// still leave a measure worth reading.
+    static func previewIndent(bodyPointSize: CGFloat) -> CGFloat {
+        (bodyPointSize * 1.5).rounded()
+    }
+
+    /// The inside padding of a table cell and the inset of a code block — half a gutter, so the
+    /// rendered body's rhythm is the column's rhythm halved rather than a second, unrelated one.
+    static func previewPadding(bodyPointSize: CGFloat) -> CGFloat {
+        (gutter(bodyPointSize: bodyPointSize) / 2).rounded()
+    }
+
+    /// The widest an inline image is drawn at.
+    ///
+    /// A number rather than "the text container's width" on purpose: a `NSTextAttachment`'s bounds
+    /// are fixed at build time, so an image sized to the window would have to be rebuilt on every
+    /// resize — and the body column is resizable by contract. A generous cap keeps a screenshot
+    /// legible without letting a 4000-pixel-wide one push the measure around, and an image narrower
+    /// than the cap is never enlarged.
+    static func previewImageMaximumWidth(bodyPointSize: CGFloat) -> CGFloat {
+        columnWidth(characters: 56, bodyPointSize: bodyPointSize)
+    }
+
     // MARK: - The window
 
     /// The window's minimum size: the sidebar's fixed width plus the body's floor, and tall enough