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>

README.md

@@ -76,6 +76,11 @@ Each row has the match status and type, a recommendation, both paths, where link
 bit depth and duration for both sides, the quality verdict and any notes. The file opens cleanly in Excel, Numbers and LibreOffice
 (UTF-8, and cells a spreadsheet would treat as formulas are escaped).
 
+**Listening in build mode.** Every song in the Build tab has a play button. A mini-player along the bottom of the page plays the
+folder in order, with seeking, previous/next, volume and space to pause. Songs are streamed from the master library as they are
+(nothing is converted), so they play when the browser supports the format: FLAC, MP3, AAC/M4A, Ogg/Opus and WAV in current browsers,
+while formats such as WMA or APE show a message instead.
+
 **Folders without music** (the *Cleanup* tab). Choose the slave or master library and **Find folders**: Playlink lists the top-most
 folders with no music file anywhere inside them (music = the pair's file extensions), such as album folders left empty after linking or
 ones holding only a cover or playlist, with what they still contain. Empty folders are pre-selected, folders that still hold files are