Add system tray with minimize-to-tray (Tartube parity 1.6)

Uses ksni (StatusNotifierItem via zbus) so we keep our pure-Rust /
no-GTK stack — zbus is already pulled in by notify-rust and rfd.

Behavior:
- Tray icon shows the bundled icon.png; left-click brings the window
  back to focus
- Right-click menu: Show / Hide / Quit
- Clicking the window's X button minimizes to tray instead of exiting
- Ctrl+Q (or the tray's Quit item) actually exits

Failure modes are silent and non-blocking: no DBus → app behaves as if
tray flag was off. No StatusNotifier host (e.g. GNOME without the
AppIndicator extension) → icon invisible but app still runs normally,
and the X-button close cancellation only fires when the tray spawn
returned a handle.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Cargo.lock

@@ -2040,6 +2040,19 @@ version = "3.1.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "e2db585e1d738fc771bf08a151420d3ed193d9d895a36df7f6f8a9456b911ddc"
 
+[[package]]
+name = "ksni"
+version = "0.3.4"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "a7ca513d0be42df5edb485af9f44a12b2cb85af773d91c27dc796d1c58b78edc"
+dependencies = [
+ "futures-util",
+ "pastey",
+ "serde",
+ "tokio",
+ "zbus 5.15.0",
+]
+
 [[package]]
 name = "leb128fmt"
 version = "0.1.0"
@@ -2731,6 +2744,12 @@ version = "1.0.15"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "57c0d7b74b563b49d38dae00a0c37d4d6de9b432382b2892f0574ddcae73fd0a"
 
+[[package]]
+name = "pastey"
+version = "0.2.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "2ee67f1008b1ba2321834326597b8e186293b049a023cdef258527550b9935b4"
+
 [[package]]
 name = "percent-encoding"
 version = "2.3.2"
@@ -5104,6 +5123,7 @@ dependencies = [
  "axum",
  "eframe",
  "image",
+ "ksni",
  "notify-rust",
  "r2d2",
  "r2d2_sqlite",