docs(release-1.2.8): Mixtapes & Collections tag notes

Release notes for 1.2.8 explaining Mixtape vs Collection vs Playlist in
user-facing terms: playlists are Qobuz-hosted track lists, Mixtapes are
private ordered cross-source listening sessions, Collections are
private themed album shelves. Covers the Discography Builder, the
Add to Mixtape picker kind rules, custom covers, duplicate-handling
dialog, and the under-the-hood notes for upgraders (new tables, new
v2_* command surface, Plex queue resolver branch).

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docs/release-1.2.8/TAG_DETAILS.md

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+# 1.2.8 — Mixtapes & Collections
+
+A new way to curate your listening that lives entirely inside QBZ — no cloud
+round-trips, no sync dance, and crucially no "tracks from Qobuz only"
+restriction. Two first-class concepts land alongside the existing Qobuz
+playlists: **Mixtapes** and **Collections**. Both are independent from
+Qobuz's own playlist system, and once you've used them for a week you'll
+notice they fill different gaps.
+
+---
+
+## Mixtape vs. Collection vs. Playlist
+
+Three container types now coexist. They look similar on the surface but
+each one serves a different need. Picking the right one is the difference
+between building a catalogue and building a listening session.
+
+### Playlist (the Qobuz one — unchanged)
+
+A **cloud-native track list, owned by your Qobuz account**.
+
+- The atomic unit is a **single track**.
+- Lives on Qobuz's servers. Installs of QBZ on other machines see the
+  same playlists because they're tied to your account.
+- Everything in it has to exist in the **Qobuz catalogue** — no room for
+  a local FLAC or a Plex track.
+- Order is loose; shuffle is a first-class operation.
+- Mental model: *"My favourites from the 80s"*, *"Discoveries this month"*.
+
+### Mixtape (new)
+
+A **private, cross-source, ordered listening session**.
+
+- The atomic unit is **any object**: a whole album, a single track, or
+  even a Qobuz playlist nested inside.
+- Lives **only in your QBZ install** — nothing leaves the machine, nothing
+  syncs to Qobuz.
+- **Cross-source by design**: a Qobuz album can sit right next to a FLAC
+  from your drive, followed by an album from your Plex library. No other
+  container in the app can do this.
+- **Order matters**. A Mixtape is meant to be played from start to finish
+  — the cassette metaphor. Small and curated (five, ten items), not a
+  catalogue.
+- Mental model: *"Music for the drive to Valle"*, *"What I'm going to
+  show my cousin"*, *"Saturday training tape"*.
+
+### Collection (new)
+
+A **large themed album shelf**.
+
+- The atomic unit is **a whole album only** — no loose tracks, no nested
+  playlists.
+- Grows as large as you want: 20, 50, 100 albums.
+- Order is softer than a Mixtape. Reproduction supports **album-shuffle**
+  (you get a random album from the shelf, but its tracks still play in
+  order from the top — not shuffled within themselves).
+- Mental model: *"Everything Macross"*, *"2000s progressive metal"*,
+  *"Radiohead's full discography"* — the last one has its own automated
+  flavour we call an **Artist Collection**, built in one click by the
+  new **Discography Builder**.
+
+### In one line
+
+- **Playlist** = a Qobuz-hosted track list.
+- **Collection** = a private themed album shelf.
+- **Mixtape** = a private, ordered, cross-source listening session.
+
+The **Add to Mixtape/Collection** picker knows the difference: when you
+add an album you can target either; when you add a track or a playlist,
+only Mixtapes show up (a "shelf of albums" with a loose track inside
+wouldn't make sense).
+
+---
+
+## Where to find them
+
+- New entries in the sidebar under **My QBZ** (collapsible).
+- **Add to Mixtape/Collection** is wired into every context menu where a
+  track, album, or playlist can be actioned: Local Library, Artist page,
+  Search results, Label view, Playlist detail, Album detail, the queue's
+  item rows.
+- Multi-select works in Local Library albums, in Mixtape/Collection
+  detail rows, and inside the Discography Builder — paired with the
+  bulk action bar so you can queue, add to playlist, or add to another
+  Mixtape/Collection in one pass.
+
+---
+
+## Building an Artist Collection the easy way
+
+For the common case of *"give me every Radiohead album in one shelf"*,
+there's a new **Discography Builder** on every artist page. It fans out
+across Qobuz **and** your local library **and** your Plex cache, groups
+identical titles, lets you override the release type (Album / EP /
+Single / Live / Compilation) when the auto-detection gets it wrong, and
+stamps the result as an Artist Collection you can play or shuffle in
+one click.
+
+---
+
+## Under the hood (for upgraders)
+
+- New tables: `mixtape_collections`, `mixtape_collection_items` — with
+  CASCADE deletion and additive migrations.
+- New Tauri command surface: `v2_*_mixtape_*` for CRUD, `v2_enqueue_collection`
+  for playback, `v2_skip_to_next_item` / `previous_item` for hop-by-album
+  navigation inside a Mixtape, `v2_mixtape_item_exists` for duplicate
+  pre-checks, `v2_mixtape_upload_custom_cover` / `remove_custom_cover`
+  for user artwork.
+- Duplicate handling is explicit: if you try to add an item that's
+  already in the target, you get a confirmation dialog with **Add
+  anyway** / **Skip duplicates** / **Cancel** — same pattern as the
+  existing Qobuz-playlist duplicate check.
+- Per-user cosmetic override for release-type labels (Album / EP /
+  Single / Live / Compilation) in a local sidecar, never written back
+  to Qobuz or the library DB.
+- The Plex branch of the queue resolver routes `plex:` album keys to
+  the Plex cache instead of `local_tracks`, so a Collection that starts
+  with a Plex album plays correctly end-to-end.
+
+---
+
+Full changelog: (link added at tag time)