Add mise tasks, desktop entry, and app icon

mise run install builds a release binary and installs it into ~/.local with
a desktop entry and a hicolor icon.

Two wrinkles specific to a Nix-built GTK app installed outside the dev shell:
the launcher is a generated script rather than a symlink, because a desktop
launcher provides a minimal environment and GTK still needs to be pointed at
its GSettings schemas and icon themes; and install registers Nix GC roots for
every store path in the binary's RPATH, so nix-collect-garbage cannot delete
GTK out from under the installed app.

The runtime paths are exported from the flake's shellHook and read back at
install time, so they cannot drift from what the binary was built against.

dist/dev.greyson.vtabs.desktop.in

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+[Desktop Entry]
+Type=Application
+Name=vtabs
+GenericName=Terminal
+Comment=Terminal emulator with vertical tabs
+# Substituted with an absolute path by `mise run install`, because desktop
+# launchers do not reliably have ~/.local/bin on PATH.
+Exec=@EXEC@
+Icon=dev.greyson.vtabs
+Terminal=false
+Categories=System;TerminalEmulator;
+Keywords=shell;prompt;command;commandline;cmd;terminal;tabs;
+StartupNotify=true
+# Lets the compositor match the running window to this entry, which is what
+# gives the window its icon. Must equal the GApplication id.
+StartupWMClass=dev.greyson.vtabs

dist/dev.greyson.vtabs.svg

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+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+  vtabs app icon: a terminal pane with the vertical tab strip down its left
+  edge. Shapes are kept large and high-contrast so the sidebar is still
+  readable when the icon is scaled down to 32px in a launcher.
+-->
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 128 128" width="128" height="128">
+  <defs>
+    <linearGradient id="pane" x1="0" y1="0" x2="0" y2="1">
+      <stop offset="0" stop-color="#241f31"/>
+      <stop offset="1" stop-color="#14111b"/>
+    </linearGradient>
+    <linearGradient id="accent" x1="0" y1="0" x2="1" y2="1">
+      <stop offset="0" stop-color="#c7b3ff"/>
+      <stop offset="1" stop-color="#9b7ff0"/>
+    </linearGradient>
+    <!-- Everything is clipped to the rounded body so the sidebar picks up
+         the same corner radius without repeating the path. -->
+    <clipPath id="body">
+      <rect x="8" y="12" width="112" height="104" rx="24"/>
+    </clipPath>
+  </defs>
+
+  <rect x="8" y="12" width="112" height="104" rx="24" fill="url(#pane)"/>
+
+  <g clip-path="url(#body)">
+    <rect x="8" y="12" width="42" height="104" fill="#2f2743"/>
+    <rect x="49" y="12" width="2" height="104" fill="#3d3357"/>
+
+    <!-- Tab rows: the active one carries the accent, the rest are muted. -->
+    <rect x="16" y="30" width="26" height="12" rx="6" fill="url(#accent)"/>
+    <rect x="16" y="50" width="26" height="12" rx="6" fill="#5d5280"/>
+    <rect x="16" y="70" width="26" height="12" rx="6" fill="#493f66"/>
+  </g>
+
+  <!-- Prompt chevron and cursor, the universal shorthand for a terminal. -->
+  <path d="M66 51 L79 64 L66 77"
+        fill="none" stroke="#e8e2f7" stroke-width="9"
+        stroke-linecap="round" stroke-linejoin="round"/>
+  <rect x="86" y="70" width="22" height="8" rx="4" fill="url(#accent)"/>
+
+  <rect x="8" y="12" width="112" height="104" rx="24"
+        fill="none" stroke="#000000" stroke-opacity="0.28" stroke-width="2"/>
+</svg>