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.

package-lock.json

@@ -1,12 +1,12 @@
 {
-  "name": "soulseekplus",
-  "version": "0.1.0",
+  "name": "soulstream",
+  "version": "0.1.0-alpha.1",
   "lockfileVersion": 3,
   "requires": true,
   "packages": {
     "": {
-      "name": "soulseekplus",
-      "version": "0.1.0",
+      "name": "soulstream",
+      "version": "0.1.0-alpha.1",
       "license": "ISC",
       "dependencies": {
         "@tauri-apps/api": "^2"