feat(client/linux): the app icon reaches About — and the launcher, which never had it either

Adding the icon to the About dialog turned out to be one line and a missing install: the
icon existed, but only the flatpak ever shipped it. The deb, rpm, arch and nix entries all
carried `Icon=video-display` — a stock monitor glyph — so on every non-flatpak install the
launcher, the taskbar and the window switcher have been showing a generic icon this whole
time. About would have shown the same nothing.

So the icon moves out of `packaging/flatpak/` into `packaging/linux/icons/hicolor/` beside
the tray icons, which is where a shared asset belongs, and all five packagings install it
to the same hicolor path. The desktop entry names the app's own icon, and `AboutDialog`
names the app id — one identity resolved through the icon theme rather than a path anyone
has to keep in step.

A `PUNKTFUNK_SHOT_SCENE=about` scene comes along so the dialog is capturable like every
other surface; that is how the icon above was verified to actually resolve rather than
silently falling back.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

packaging/linux/icons/hicolor/scalable/apps/io.unom.Punktfunk.svg

@@ -0,0 +1,12 @@
+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg width="1000" height="1000" viewBox="0 0 1000 1000" version="1.1"
+     xmlns="http://www.w3.org/2000/svg"
+     style="fill-rule:evenodd;clip-rule:evenodd;stroke-linejoin:round;stroke-miterlimit:2;">
+  <!-- Brand mark: three overlapping circles (flattened from clients/apple punktfunk_Logo.icon).
+       Order back-to-front: large violet circle (layer 1), deep-purple circle (layer 2),
+       light lens highlight (layer 3). -->
+  <rect x="0" y="0" width="1000" height="1000" rx="180" ry="180" fill="#1c1530"/>
+  <path d="M403.037,791.672c107.586,0 194.41,-86.824 194.41,-194.41c0,-107.586 -86.824,-194.41 -194.41,-194.41c-107.586,0 -194.41,86.824 -194.41,194.41c0,107.586 86.824,194.41 194.41,194.41Z" fill="#a79ff8"/>
+  <path d="M735.276,540.321c76.075,-76.075 76.075,-198.862 0,-274.937c-76.075,-76.075 -198.862,-76.075 -274.937,0c-76.075,76.075 -76.075,198.862 0,274.937c76.075,76.075 198.862,76.075 274.937,0Z" fill="#6c5bf3"/>
+  <path d="M647.84,590.737c-64.853,17.403 -136.871,0.597 -187.885,-50.416c-51.013,-51.013 -67.819,-123.032 -50.416,-187.885c64.853,-17.403 136.871,-0.597 187.885,50.416c51.013,51.013 67.819,123.032 50.416,187.885Z" fill="#d2c9fb"/>
+</svg>

packaging/linux/io.unom.Punktfunk.desktop

@@ -3,7 +3,7 @@ Type=Application
 Name=Punktfunk
 Comment=Stream a remote punktfunk host
 Exec=punktfunk-client %u
-Icon=video-display
+Icon=io.unom.Punktfunk
 Terminal=false
 Categories=Network;Game;
 Keywords=streaming;remote;game;moonlight;