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

Cargo.lock

@@ -1826,6 +1826,29 @@ dependencies = [
  "winit",
 ]
 
+[[package]]
+name = "egui_commonmark"
+version = "0.25.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "19e757b2a0639f5f75793e6936ad4c86e8b58ee379d23578ad6924ba9f75ec38"
+dependencies = [
+ "egui",
+ "egui_commonmark_backend",
+ "egui_extras",
+ "pulldown-cmark",
+]
+
+[[package]]
+name = "egui_commonmark_backend"
+version = "0.25.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "67e182e6b923b0ade1152f67e7d827cbc266fca7782d50249de0c4cad1a2a4e5"
+dependencies = [
+ "egui",
+ "egui_extras",
+ "pulldown-cmark",
+]
+
 [[package]]
 name = "egui_extras"
 version = "0.36.2"
@@ -3887,6 +3910,7 @@ dependencies = [
  "eframe",
  "egui",
  "egui-phosphor",
+ "egui_commonmark",
  "egui_extras",
  "egui_kittest",
  "hayro 0.8.0",
@@ -5423,6 +5447,17 @@ dependencies = [
  "cc",
 ]
 
+[[package]]
+name = "pulldown-cmark"
+version = "0.13.5"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "278e9268c93b9e498a2da46c34d90ba93a3255eb99009646398f34bc73ab36eb"
+dependencies = [
+ "bitflags 2.13.2",
+ "memchr",
+ "unicase",
+]
+
 [[package]]
 name = "pxfm"
 version = "0.1.30"