fix(local-library): use high-positive ids for ephemeral tracks (queue/context u64 compat)

The original design used negative i64 ids to distinguish ephemeral
tracks from DB rows, but the queue and playback-context commands
serialize ids as Vec<u64> end-to-end (V2QueueTrack, v2_set_playback_context).
serde rejects negative numbers at the boundary, so setQueueForLocalTracks
and setPlaybackContext failed the moment the user clicked an ephemeral
track:

    invalid value: integer `-1`, expected u64

Switch synthetic ids to a high-positive range (EPHEMERAL_ID_FLOOR =
1<<48) instead. Pros over the negative scheme:
* Valid u64 — passes through the queue/context commands untouched
* Below 2^53 so JS Number round-trips without precision loss
* Far above any realistic DB autoincrement id, so the discrimination
  test (`track_id >= EPHEMERAL_ID_FLOOR`) stays unambiguous

Also fixes the snake_case mismatch on EphemeralFolderState — the Rust
struct serializes with its native field names (folder_path,
skipped_files), but the TypeScript type was declared in camelCase, so
ephemeralFolder.folderPath was undefined at render time.

cc

src-tauri/src/commands_v2/library.rs

@@ -839,12 +839,15 @@ pub async fn v2_library_play_track(
         .await
         .map_err(|e| e.to_string())?;
 
-    // Negative ids belong to the ephemeral library (an ad-hoc folder the
-    // user opened without persisting it to local_tracks). Resolve to the
-    // in-memory cache and skip every DB-bound branch below — ephemeral
-    // tracks are never qobuz_download, never offline-cached, never have a
-    // CUE pointer. Just read the file and hand it to the player.
-    if track_id < 0 {
+    // Track ids at or above EPHEMERAL_ID_FLOOR belong to the ephemeral
+    // library (an ad-hoc folder the user opened without persisting it
+    // to local_tracks). Resolve to the in-memory cache and skip every
+    // DB-bound branch below — ephemeral tracks are never qobuz_download,
+    // never offline-cached, never have a CUE pointer. Just read the
+    // file and hand it to the player. The high-positive id range keeps
+    // the existing u64-typed queue/context commands happy while still
+    // being unambiguous (DB autoincrement ids never reach 2^48).
+    if track_id >= crate::ephemeral_library::EPHEMERAL_ID_FLOOR {
         let track = ephemeral_state
             .get_track(track_id)
             .ok_or_else(|| format!("Ephemeral track {} not found (session may have been cleared)", track_id))?;