feat(slint): first-class local playlists with offline-only mode (D7/D8)

New local_playlists + local_playlist_tracks schema in the shared per-user
library.db (ids 'local:<uuid>', sources qobuz/local/plex, position-compacted,
8 repo tests in qbz-library). PlaylistRef routing at the navigation seam
makes local ids unrepresentable in Qobuz playlist calls by construction
(all take u64). Local playlists appear in sidebar, manager and picker
(picker hides the Qobuz set while offline); detail view reuses the existing
playlist row machinery and renders offline; creation modal gains the
offline-only toggle (forced on while offline - creating offline always
produces a local playlist, D8) and Edit can flip it later; explicit
Upload-to-Qobuz conversion for non-offline-only locals (confirm dialog,
local rows attach via the existing sidecar, entity kept on failure).

Never-to-Qobuz guard: queues built from an offline-only playlist carry a
core-level stamp cleared by every queue replacement; the QConnect sync
skips the cloud push and play-on-peer falls back to local playback while
the stamp is set.

Deferred (logged in the slice report): plex-row attach on upload, custom
reorder for local playlists, folder/favorite/hide on local items, manager
sort interleaving, offline cover thumbs for cached qobuz rows.

cc

crates/qbz-slint/ui/state.slint

@@ -964,6 +964,9 @@ export struct SidebarEntry {
     count: int,         // folder playlist count
     indent: bool,       // playlist nested in a folder
     folder-id: string,  // playlist's current folder id ("" = root; folders "")
+    // "" (Qobuz) | "local" | "offline" — local playlists (library.db,
+    // ids "local:<uuid>") show a hard-drive glyph instead of list-music.
+    local-kind: string,
     cover-count: int,   // number of cover images, 0-4 (playlists)
     url1: string,
     url2: string,
@@ -1094,6 +1097,12 @@ export global CreatePlaylistState {
     in property <[string]> folder-ids: [""];
     // Selected index into `folder-options` (0 = No folder).
     in-out property <int> folder-index: 0;
+    // D8 "Offline-only playlist" opt-in: ON -> created as a LOCAL entity
+    // (library.db, id "local:<uuid>") that never reaches Qobuz.
+    in-out property <bool> offline-only: false;
+    // True while the app is offline: creation always produces a local
+    // playlist, so the toggle shows ON and locked with a hint.
+    in property <bool> offline-locked: false;
 }
 
 export global CreatePlaylistActions {
@@ -1240,6 +1249,12 @@ export global PlaylistState {
     in property <bool> is-favorite: false;
     // True when the playlist is one of the user's own (editable).
     in property <bool> is-owner: false;
+    // LOCAL playlist (id "local:<uuid>", library.db entity — never on
+    // Qobuz). Qobuz-bound header actions (favorite / cache) hide; the
+    // "Upload to Qobuz" action appears when not offline-only.
+    in property <bool> is-local: false;
+    // D8: never offered for upload, never reaches Qobuz/QConnect.
+    in property <bool> offline-only: false;
     // Track edit mode — rows show a selection checkbox and a bulk bar
     // appears (remove selected). Only available to the owner.
     in-out property <bool> multi-select-mode: false;
@@ -1266,6 +1281,12 @@ export global EditPlaylistState {
     in-out property <string> name;
     in-out property <string> description;
     in property <bool> busy: false;
+    // LOCAL playlist (id "local:<uuid>") — edits route to library.db and
+    // the offline-only toggle below shows.
+    in property <bool> is-local: false;
+    // D8 flag, editable so an offline-created playlist can be unmarked
+    // later and gain the "Upload to Qobuz" action.
+    in-out property <bool> offline-only: false;
 }
 
 export global EditPlaylistActions {
@@ -1321,6 +1342,10 @@ export struct PmPlaylistItem {
     local-status: string,
     is-favorite: bool,
     is-hidden: bool,
+    // LOCAL playlist entity (id "local:<uuid>", library.db) — the card /
+    // list-row shows a hard-drive marker; offline-only flavors label it.
+    is-local-playlist: bool,
+    offline-only: bool,
     folder-id: string,     // "" = root
     // Micro-collage covers (same 0-4 scheme as the sidebar).
     cover-count: int,
@@ -2895,6 +2920,9 @@ export struct PlaylistPickItem {
     id: string,
     name: string,
     tracks-line: string,   // "N tracks"
+    // LOCAL playlist (library.db entity, id "local:<uuid>") — the row
+    // shows a hard-drive glyph; adds write the local repo (works offline).
+    is-local: bool,
 }
 
 // "Add to playlist" picker — a global overlay opened from the track