v0.243.0: m Scale and zoom each message body alone, never the reader's chrome

One fixed-width signature table (997px in the wild) made the fit-to-width
pass scale the whole conversation — header bands, gist cards, and the
thread's other messages shrank with it. Each body now lives in its own
.mbfit unit the shell scales independently, so the app's chrome always
renders at full size.

Reader zoom moves into the same pass: __mbZoom lays each body out at
avail/zoom and scales it by exactly zoom× its fit size, replacing
WebView.SetZoomLevel, whose page zoom scaled the chrome too — the
original complaint. Reflowing mail re-wraps and never overflows; a
width-locked body grows past the pane and the page pans horizontally
(the html clip flips to auto) with a styled, always-visible scrollbar
pinned to the reader viewport's bottom edge — WebKit's hover-only
overlay on a per-body scroller sat at the bottom of the message,
screens away on a tall email, which read as "no horizontal scrolling".
The persisted zoom applies when the shell reports ready, and setZoom
snaps float drift back to exactly 1 so reset returns to the
no-transform path.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>