android: the downloads say so in the shade

A Save still running with the app in the background now posts a progress
notice — how many files, the one it is on, a bar, Stop all and Open — and
takes it down when the queue empties. It sits on its own low-importance
channel so it can be silenced without silencing the player's card, and
rides its own dataSync foreground service rather than the media one: a
mediaPlayback service stops itself when the bar empties, a download is not
playback, and nothing else was keeping the process alive for a Save with
nothing playing — which is how the first real Save died, four tracks in.
Stop all goes through the same cancel(id) a row's tap takes, and Transfers
grew the matching button beside Clear finished. The walk pushes two rows
straight into the transfer store and reads the real notification back out
of `dumpsys notification`, without fetching anything.

src/mobile/xfernote.svelte.js

@@ -0,0 +1,115 @@
+/* The downloads in the phone's shade.
+
+   The Transfers screen is inside the app: the desktop says what a Save is
+   doing with its title-bar ring, the phone with a badge on the nav, and
+   neither is anywhere to be seen once the app is in the background — which
+   is exactly when a Save is left to run on its own. So a running Save says
+   so where the phone says things: one notice with how many files are left,
+   the one it is on, a bar, Stop all and Open.
+
+   Its own channel, so it can be silenced without silencing the player's
+   card, and its own foreground service, so the fetch outlives the playback
+   that happened to be keeping the process alive. Everything real is in
+   crates/tauri-plugin-transfer-notice; this is the wiring — the mirror of
+   transfers.svelte.js, exactly as media.svelte.js mirrors the player. */
+import * as api from "../lib/api.js";
+import { addPluginListener } from "@tauri-apps/api/core";
+import { active, cancelAll } from "./transfers.svelte.js";
+import { note } from "./notice.svelte.js";
+import { go, push, top, closeSheet } from "./nav.svelte.js";
+
+/* What the phone was last told, and how often. A PLAIN object, not $state:
+   nothing renders it, and the effect that drives the post must never read
+   what it writes — a self-read in an $effect kills the subtree. The walk
+   reads it through __ss.xfn, since a notification is not a thing a headless
+   page can be asked about. */
+export const xfernote = {
+    posted: false,
+    updates: 0,
+    clears: 0,
+    max: 0,
+    title: "",
+    line: "",
+    percent: -1,
+    failed: 0,
+};
+
+/* a re-post is not free, and progress ticks arrive far faster than anyone
+   reads them: the lever is the rate */
+const MIN_GAP = 700;
+let timer = null;
+let lastAt = 0;
+let lastKey = "";
+
+/** What is in flight, as the notice would say it — or null when nothing is. */
+function shape() {
+    const live = active();
+    if (!live.length) return null;
+    const newest = [...live].sort((a, b) => b.id - a.id);   /* the screen's own order */
+    const cur = newest.find((d) => d.state === "transferring") || newest[0];
+    let received = 0, total = 0, known = 0;
+    for (const d of live) if (d.total) { received += d.received || 0; total += d.total; known++; }
+    return {
+        count: live.length,
+        title: live.length === 1 ? "Saving 1 file" : `Saving ${live.length} files`,
+        line: cur.state === "transferring" ? (cur.name || cur.path || "") : "waiting in their queue",
+        percent: total ? Math.round((received / total) * 100) : 0,
+        indeterminate: !known,
+    };
+}
+const keyOf = (s) => (s ? `${s.count}|${s.title}|${s.line}|${s.percent}` : "");
+
+/** Called from Mobile.svelte's effect: reads the store — so the effect
+ *  tracks it — and schedules at most one post per MIN_GAP. */
+export function syncShade() {
+    const key = keyOf(shape());
+    if (key === lastKey || timer) return;
+    timer = setTimeout(post, Math.max(0, MIN_GAP - (Date.now() - lastAt)));
+}
+
+/** The one place that talks to the plugin. Reads the store again on the way
+ *  out, so what lands in the shade is the latest, not the change that
+ *  scheduled it. */
+async function post() {
+    timer = null;
+    lastAt = Date.now();
+    const s = shape();
+    lastKey = keyOf(s);
+    if (!s) {
+        if (!xfernote.posted) return;
+        xfernote.posted = false;
+        xfernote.clears++;
+        try { await api.clearTransferNotice(); } catch { xfernote.failed++; }
+        return;
+    }
+    xfernote.posted = true;
+    xfernote.updates++;
+    if (s.count > xfernote.max) xfernote.max = s.count;
+    xfernote.title = s.title;
+    xfernote.line = s.line;
+    xfernote.percent = s.percent;
+    try { await api.transferNotice(s); } catch { xfernote.failed++; }
+}
+
+let bound = false;
+/** The notice's own two buttons. Harmless where there is no plugin. */
+export async function bindXferNote() {
+    if (bound) return;
+    bound = true;
+    try {
+        await addPluginListener("transfer-notice", "action", (p) => {
+            if (p?.action === "cancel") {
+                /* the same cancel(id) a row's tap takes, once per live row —
+                   never a second path to the same thing */
+                void cancelAll();
+                note("Downloads stopped.");
+            } else if (p?.action === "open") {
+                closeSheet();
+                go("you");
+                if (top().n !== "transfers") push("transfers", {});
+            }
+        });
+    } catch {
+        /* not a phone */
+    }
+}