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/LiveStore/BannerCenter.swift

@@ -641,6 +641,11 @@ public final class BannerCenter {
             // failure the user did not provoke is exactly the one they have no other way to learn
             // about.
             "Couldn't move '\(filename)' into attachments"
+        case let .toggleTask(title):
+            // The user's word for it, not the file's: they ticked a box. The card is named where
+            // the read that preceded the flip learned its title, so a body write that refused says
+            // *which* card refused it — a card window is not always the frontmost thing on screen.
+            if let title { "Couldn't tick the checkbox in '\(title)'" } else { "Couldn't tick the checkbox" }
         }
     }
 
@@ -655,6 +660,8 @@ public final class BannerCenter {
             "this file's frontmatter can't be edited in place (\(shape.description))"
         case let .io(message):
             message
+        case let .staleTarget(message):
+            message
         }
         return trimmed(text)
     }