Give the app an icon, and a script that installs it

The launcher pointed at `Icon=story-editor`, a name that only the Breeze
theme happened to ship, and only at 16 and 22 px -- so anywhere the
desktop wanted a larger icon it fell back to something generic.

assets/md-manuscript.svg is the source: a sheaf of manuscript pages, an
amber chapter-title line over three of prose. Drawn in plain rounded
rectangles because a finer design silts up into a grey smear at 16 px.
It is rasterised to the eight sizes a desktop asks for, and the 256px
PNG is compiled into the binary as the window icon, so the title bar and
the task switcher carry it whether or not anything is installed. That
costs no new dependency: eframe already pulls `image` with png decoding.
A test decodes the bundled PNG, since a rename would otherwise show up
only as a blank icon at runtime.

Installation is now `./assets/install.sh` -- binary, icons, launcher and
both cache refreshes, safe to re-run after a rebuild. The launcher is a
file in the repository rather than a here-document in the instructions:
pasting one into a terminal leaves the shell sat at a `>` prompt waiting
for a terminator that never arrives, which is exactly what happened, and
this way the launcher is versioned along with everything else.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GYSGPDwkSzhm4qLqjCbqxU

assets/icons/hicolor/scalable/apps/md-manuscript.svg

@@ -0,0 +1,14 @@
+<svg xmlns="http://www.w3.org/2000/svg" width="64" height="64" viewBox="0 0 64 64">
+  <title>md-manuscript</title>
+  <!-- App tile: ink -->
+  <rect width="64" height="64" rx="13" fill="#22304a"/>
+  <!-- A sheaf of pages: the sheet behind peeks out above and to the right -->
+  <rect x="19" y="9" width="30" height="40" rx="2.5" fill="#c9c1b0"/>
+  <rect x="14" y="14" width="30" height="40" rx="2.5" fill="#f6f1e6"/>
+  <!-- The chapter title, then prose. Few lines, widely spaced: at 16px a
+       denser block of them silts up into an illegible grey smear. -->
+  <rect x="19" y="21" width="16" height="3.4" rx="1.7" fill="#c8763c"/>
+  <rect x="19" y="30" width="20" height="3" rx="1.5" fill="#3f5578"/>
+  <rect x="19" y="37" width="20" height="3" rx="1.5" fill="#3f5578"/>
+  <rect x="19" y="44" width="12" height="3" rx="1.5" fill="#3f5578"/>
+</svg>