feat(media-controls): SMTC with a real HWND, and a native cover path

Windows media keys never worked because SMTC never started. souvlaki's Windows
backend does `config.hwnd.expect(..)`, which PANICS rather than degrading, and
`MediaControls::new` runs inside a tokio task, so a missing handle aborted the
task instead of returning the Err the caller was written for. Phase A bowed out
before reaching it and logged "SMTC disabled: no window handle wired yet".

`spawn` now takes a `NativeWindow`, and `cxx/win_shell.cpp` supplies the
top-level HWND through `QGuiApplication::topLevelWindows()`. The guard stays:
a missing integration is a missing feature, a panic in a tokio task is a dead
session.

Windows reads the handle from a bounded retry on the GUI thread. Both things it
waits for -- the tray bridge's Qt thread and a shown window -- are "not yet",
never "not ever": `tray_bridge::ui` is a no-op until `QbzTray::boot` registers
the thread, and `QbzSession.boot()` runs first (Main.qml:254 vs :380).

NON-WINDOWS IS UNCHANGED, deliberately. An earlier draft routed every platform
through that hop and it was a Linux/macOS regression three ways: initialization
could be dropped outright before boot, MPRIS would start only when the Qt queue
drained (losing the first metadata push), and `main_window_hwnd` calls
`QWindow::winId()`, which CREATES the platform window as a side effect on
platforms with no reason to.

Cover art needed its own fix. souvlaki 0.8.3 branches on
`url.starts_with("file://")`, then trims exactly that prefix and hands the rest
to `GetFileFromPathAsync`. So `file:///C:/x.jpg` arrives as `/C:/x.jpg` -- a
slash before a drive letter, which never opens -- while a bare `C:\x.jpg` no
longer matches the prefix and takes the remote-URI branch instead. The value
that works is the hybrid `file://C:\x.jpg`. UNC is handled too: `file://nas/m/a`
is a real Windows path and was being passed through as the relative `nas/m/a`.

The percent-decode is full and applied once: `art_url` arrives from
`url::Url::from_file_path` (space as %20) and from `fs_url::file_url` (only
% # ? escaped), and one full decode reads both -- `%2520` gives back a literal
`%20` rather than a space. `fs_url::local_path` is deliberately not used here;
its three-escape decode would leave the other producer's %20 intact.

Eight tests cover the matrix. Reviewed adversarially before commit, which is
where the inverted cover conversion and the non-Windows regression were caught;
the souvlaki branch was then read at the source to confirm.

cc

crates/Cargo.lock

@@ -4367,10 +4367,12 @@ dependencies = [
  "md-5",
  "mpris-server",
  "notify-rust",
+ "qbz-models",
  "reqwest",
  "souvlaki",
  "tokio",
  "urlencoding",
+ "windows-sys 0.61.2",
 ]
 
 [[package]]
@@ -4581,6 +4583,7 @@ dependencies = [
  "tokio",
  "urlencoding",
  "uuid 1.22.0",
+ "windows-sys 0.61.2",
  "winresource",
  "zbus 5.15.0",
 ]