feat(app): turn on AccessKit, and name the controls drawn as icons

eframe now builds with its accesskit feature, so the widget tree reaches
the platform screen reader. Buttons drawn as an icon glyph were read as
the icon font's private code point, and the hand-painted ones (play,
poster tiles, the watched mark, download rings, the seek bar) as
nothing. A Named trait gives them a name, and hint() makes the existing
hover text that name.

Cargo.toml

@@ -73,8 +73,10 @@ keyring = { version = "3" }
 clap = { version = "4", features = ["derive"] }
 # The UI. `glow` rather than `wgpu` because step two of the player hands mpv an
 # `MPV_RENDER_API_TYPE_OPENGL` context, and that has to be the same context
-# eframe is drawing into.
+# eframe is drawing into. `accesskit` hands the widget tree to the platform's
+# screen reader (AT-SPI, UI Automation, NSAccessibility).
 eframe = { version = "0.35", default-features = false, features = [
+    "accesskit",
     "default_fonts",
     "glow",
     "wayland",