fix: previews follow the app volume; volume bar fill always accent

The preview backend never received a volume — clips played at the crate
default (0.7) regardless of the slider, even muted. The session now sets
the preview volume from the app store (mute → 0) before the first load and
mirrors it live while the modal is open; the debug overlay's PREVIEW row
exposes the backend's resting volume as proof.

The player-bar volume (and fallback seek) fill is now always the accent
color — matching the waveform scrubber's played region — stepping up to
the hover shade on hover instead of white-until-hover.

Also syncs package-lock to the soulstream name + alpha version.

tests/preview/service.test.mjs

@@ -72,6 +72,9 @@ function makeTransport() {
         async seek(ms) {
             log.push(["seek", ms]);
         },
+        async setVolume(v) {
+            log.push(["setVolume", v]);
+        },
         async stop() {
             ctx = null;
             log.push(["stop"]);
@@ -259,6 +262,17 @@ test("auto-play: the LAST playable track never advances early", async () => {
     assert.equal(service.getState().phase, PHASE.ENDED);
 });
 
+test("setVolume forwards the app volume to the transport", async () => {
+    const { service, transport, manifest } = makeService();
+    await service.open({ sessionId: SID, manifest });
+    service.setVolume(0.35);
+    service.setVolume(0); // app mute → silent previews
+    assert.deepEqual(
+        transport.log.filter((e) => e[0] === "setVolume"),
+        [["setVolume", 0.35], ["setVolume", 0]]
+    );
+});
+
 // ------------------------------------------------------- resume memory ----
 
 test("resume: stepping back to a track picks up where it left off", async () => {