feat(tray): use ksni on Linux for working left-click toggle (#310)

Tauri's default tray uses libayatana-appindicator, which by design does
not dispatch primary-click events to its client — libayatana delegates
every left-click to the libdbusmenu-managed menu. The existing
show_menu_on_left_click(false) setting and TrayIconEvent::Click handler
never fire on Linux. Confirmed present in tray-icon 0.21.x and 0.22.0
(the newest release).

Switch the Linux backend to ksni, a pure-Rust StatusNotifierItem
implementation that exposes the full SNI protocol including Activate
(left-click), SecondaryActivate (middle), and ContextMenu. Same SNI
compatibility as the previous backend — works wherever the panel
already handled our tray (KDE, GNOME+AppIndicator, XFCE/Cinnamon/MATE,
wlroots trays). Flatpak just needs the --talk-name=org.kde.
StatusNotifierWatcher permission which is already in our manifest.

Tauri's tray stays on macOS (no Windows client is shipped). The
dispatch lives in tray.rs behind cfg(target_os); Linux delegates to
the new tray_linux_ksni module. Playback-control menu events
(tray:play_pause, tray:next, tray:previous) still emit into the main
webview window, so frontend handlers keep working unchanged.

cc

src-tauri/Cargo.toml

@@ -157,6 +157,11 @@ futures-util = "0.3"
 tauri-plugin-dialog = { version = "2.7.0" }
 ashpd = { version = "0.13", default-features = false, features = ["tokio", "notification", "inhibit", "secret"] }
 mupdf = { version = "0.6", features = ["sys-lib-zlib", "sys-lib-harfbuzz", "sys-lib-freetype"] }
+# Linux tray via StatusNotifierItem with primary-activate support.
+# Tauri's built-in tray uses libayatana-appindicator which does not dispatch
+# left-click events to clients (by design — primary click always shows the
+# menu). ksni is a pure-Rust SNI impl that exposes Activate. See issue #310.
+ksni = { version = "0.3", features = ["blocking"] }
 
 [target.'cfg(target_os = "macos")'.dependencies]
 tauri-plugin-dialog = { version = "2.7.0" }