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>

clients/linux/src/cli.rs

@@ -531,6 +531,9 @@ pub fn run_shot(ctx: &ShotCtx, scene: &str) {
                 "00aabbccddeeff112233445566778899a0b1c2d3e4f5061728394a5b6c7d8e9f",
             )));
         }
+        "about" | "08-about" => {
+            crate::ui_settings::show_about(&ctx.window);
+        }
         "settings" | "03-settings" => {
             // Mock devices so the shot shows the probe-dependent pickers populated.
             let dev = |name: &str, description: &str| pf_client_core::audio::AudioDevice {