docs: refresh readme features and download channels

The features section last changed substantively in 2c54baca3 (2026-05-17),
before the music and Explore subsystems shipped, so two whole feature areas
were missing and several availability notes had drifted.

Adds Music and Explore & Requests sections, and corrects claims that no
longer hold: the locale count (14 -> 21), the EPG guide is not Plex-only,
downloads include music and are unavailable on tvOS, Picture-in-Picture
excludes the TV platforms, and shaders and ambient lighting need the mpv
backend. Footnotes move from numeric to named so adding one no longer
renumbers the rest.

Windows now points at the Microsoft Store listing instead of the direct
installer and portable archives. The App Store and Play badges carry
campaign tokens so README traffic is attributable in each store's own
reporting.

The prerequisite Flutter version matches the pinned toolchain (3.44.0), and
the Maestro end-to-end suite gets the pointer it never had.

assets/readme_icons/explore.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><defs><linearGradient id="g" x1="0" y1="24" x2="24" y2="0" gradientUnits="userSpaceOnUse"><stop offset="0" stop-color="#ab543a"/><stop offset="1" stop-color="#ff7e57"/></linearGradient></defs><path fill="url(#g)" d="M19.3 16.9c.4-.7.7-1.5.7-2.4c0-2.5-2-4.5-4.5-4.5S11 12 11 14.5s2 4.5 4.5 4.5c.9 0 1.7-.3 2.4-.7l3.2 3.2l1.4-1.4zm-3.8.1c-1.4 0-2.5-1.1-2.5-2.5s1.1-2.5 2.5-2.5s2.5 1.1 2.5 2.5s-1.1 2.5-2.5 2.5M12 20v2C6.48 22 2 17.52 2 12S6.48 2 12 2c4.84 0 8.87 3.44 9.8 8h-2.07A8 8 0 0 0 15 4.59V5c0 1.1-.9 2-2 2h-2v2c0 .55-.45 1-1 1H8v2h2v3H9l-4.79-4.79C4.08 10.79 4 11.38 4 12c0 4.41 3.59 8 8 8"/></svg>

assets/readme_icons/music.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><defs><linearGradient id="g" x1="0" y1="24" x2="24" y2="0" gradientUnits="userSpaceOnUse"><stop offset="0" stop-color="#ab543a"/><stop offset="1" stop-color="#ff7e57"/></linearGradient></defs><path fill="url(#g)" d="M20 2H8c-1.1 0-2 .9-2 2v12c0 1.1.9 2 2 2h12c1.1 0 2-.9 2-2V4c0-1.1-.9-2-2-2m-2 5h-3v5.5a2.5 2.5 0 0 1-5 0a2.5 2.5 0 0 1 2.5-2.5c.57 0 1.08.19 1.5.51V5h4zM4 6H2v14c0 1.1.9 2 2 2h14v-2H4z"/></svg>