feat(viewer): Markdown and plain-text documents are drawn as text

A text/markdown file (or text/x-markdown) is drawn as formatted text by
egui_commonmark, with no image loading, so a document never makes the
viewer read a file or the network; any other text/* type as its
selectable text. UTF-8 only (a leading byte-order mark dropped), at most
2 MiB; past that or not UTF-8, the viewer says why. The documents page's
tile shows the text's first lines. The SHA-256 row is cut to its column
(it ran off the panel and widened the page under it).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01T3gXMVt7XJPi6ens1or9Dj

crates/ledger-gui/Cargo.toml

@@ -21,6 +21,9 @@ eframe = { version = "0.36", default-features = false, features = ["default_font
 egui = "0.36"
 egui_extras = { version = "0.36", features = ["image", "datepicker", "serde"] }
 egui-phosphor = "0.14"
+# Markdown documents, drawn as text. No image loading (no `load-images`,
+# no `fetch`): a document never makes the viewer read a file or the network.
+egui_commonmark = { version = "0.25", default-features = false, features = ["pulldown_cmark"] }
 hayro = { version = "0.8", features = ["embed-fonts"] }
 image = { version = "0.25", default-features = false, features = ["png", "jpeg", "webp"] }
 notify = "8"