android: a playlist's cover from a photo on the phone

The storage plugin gains pick_image — the document picker limited to
images, the picked content copied into the app's cache so the Rust side
gets a plain path. playlist_cover_set(id, path) is the desktop pick's
install step split out and shared (squared, the master cover, the row's
revision moved). A playlist's sheet offers Choose cover photo and Remove
custom cover; the mock walk exercises both.

tests/mobile/smoke.mjs

@@ -111,6 +111,7 @@ const PROBE = `(function () {
         plkind: (document.querySelector('.rec-head.pl .kind') || {}).textContent || '',
         plfilter: !!document.querySelector('.plfilter input'),
         plfirst: (document.querySelector('.pl-rows .trk .t') || {}).textContent || '',
+        plcover: !!document.querySelector('.rec-head.pl .rec-cover img'),
         qfirst: (document.querySelector('.q-face .row .t') || {}).textContent || '',
         rooms: document.querySelectorAll('.screen button.row .art.hash').length,
         plthumbs: document.querySelectorAll('.pl-rows .thumb img').length,
@@ -307,6 +308,11 @@ const MOCK_STEPS = [
     { do: `__click('.halfsheet .sheet-rows .row', 'Track 4')`, name: "playlist-matched", wait: 1000 },
     { do: `document.querySelectorAll('.gbar .iconbtn')[1].click()`, name: "playlist-more", wait: 700 },
     { do: `__click('.halfsheet .row', 'Unpin')`, name: "playlist-unpinned", wait: 900 },
+    /* the cover: the custom one removed (the mosaic), then a photo chosen */
+    { do: `document.querySelectorAll('.gbar .iconbtn')[1].click()`, wait: 600 },
+    { do: `__click('.halfsheet .row', 'Remove custom cover')`, name: "cover-removed", wait: 1000 },
+    { do: `document.querySelectorAll('.gbar .iconbtn')[1].click()`, wait: 600 },
+    { do: `__click('.halfsheet .row', 'Choose cover photo')`, name: "cover-set", wait: 1000 },
     { do: `document.querySelectorAll('.gbar .iconbtn')[1].click()`, wait: 600 },
     { do: `__click('.halfsheet .row', 'Rename')`, name: "playlist-rename", wait: 700 },
     { do: `(function () { var i = document.querySelector('.halfsheet .prompt .field'); i.value = 'Late night drive II'; i.dispatchEvent(new Event('input', { bubbles: true })); })()`, wait: 300 },
@@ -476,6 +482,8 @@ const MOCK_EXPECT = {
     "playlist-matched": (i) => !i.half.includes("up") && !/unresolved/.test(i.pltags) && /Linked to/.test(i.notice),
     "playlist-more": (i) => i.half.includes("up") && /Unpin/.test(i.halfText) && /Rename/.test(i.halfText) && /Delete playlist/.test(i.halfText),
     "playlist-unpinned": (i) => !i.half.includes("up") && !/Pinned/.test(i.plkind),
+    "cover-removed": (i) => !i.plcover && /mosaic/.test(i.notice),
+    "cover-set": (i) => i.plcover && /Cover set/.test(i.notice),
     "playlist-rename": (i) => i.half.includes("up") && i.promptVal === "Late night drive",
     "playlist-renamed": (i) => i.title.startsWith("Late night drive II"),
     "playlists-again": (i) => i.plcards === 3 && !i.plpinned && /Late night drive II/.test(i.plnames),