Add a mini-player to build mode

Every song in the Build tab gets a play button. A bar along the bottom
plays the folder in order with seeking, previous/next, volume (kept per
browser) and space to pause, and says so when the browser can't play a
format. Songs stream from the master library through a new endpoint with
range requests; it only serves regular music files inside the library.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/engine/listen.go

@@ -0,0 +1,48 @@
+package engine
+
+import (
+	"context"
+	"errors"
+	"fmt"
+	"os"
+	"path/filepath"
+
+	"github.com/cdnutter/playlink/internal/fsops"
+)
+
+// OpenSong opens a song in one library for listening (the build mode
+// player). Only regular music files inside the library are served: no
+// symlinks, no other file types.
+func (e *Engine) OpenSong(ctx context.Context, pairID int64, side, rel string) (*os.File, os.FileInfo, error) {
+	l, err := e.loadPair(ctx, pairID)
+	if err != nil {
+		return nil, nil, err
+	}
+	root, err := sideRoot(l, side)
+	if err != nil {
+		return nil, nil, err
+	}
+	abs, err := root.Resolve(rel)
+	if err != nil {
+		return nil, nil, err
+	}
+	if !l.isMusic(filepath.Base(abs)) {
+		return nil, nil, fmt.Errorf("%w: not a music file", ErrInvalidBulk)
+	}
+	if _, err := fsops.RegularFile(abs); err != nil {
+		return nil, nil, err
+	}
+	f, err := os.Open(abs)
+	if err != nil {
+		return nil, nil, err
+	}
+	// What was opened must still be the regular file that was checked, not
+	// something swapped in for it since.
+	fi, err := f.Stat()
+	li, lerr := os.Lstat(abs)
+	if err != nil || lerr != nil || !li.Mode().IsRegular() || !os.SameFile(fi, li) {
+		f.Close()
+		return nil, nil, errors.New("the file changed while opening it")
+	}
+	return f, fi, nil
+}