feat(qconnect): restore the device-name setting from Tauri

Tauri let the user rename how QBZ announces itself to Qobuz Connect;
the Slint port lost the UI entirely while keeping the persistence and
the announce path (same device_name key in qconnect_settings.db, read
by connect() -> bootstrap_remote_presence). Restored 1:1: a text row
in Settings > Playback next to the QConnect startup select (LineEdit
mirroring the ListenBrainz token row, placeholder = the resolved
default 'Qbz - {hostname}'), a new settings-string callback channel
through the settings chain, and a live-service setter so a rename
takes effect on the next connection without an app restart — exactly
Tauri's semantics (no live re-announce; the row description says so).
Trim-only validation like Tauri; empty clears the override. Two new
msgids across the 8 locales.

cc

crates/qbz-ui/ui/state.slint

@@ -2891,6 +2891,12 @@ export global SettingsState {
     // Rust controller, which owns the index -> mode mapping.
     in-out property <[string]> qconnect-startup-modes: [];
     in-out property <int> qconnect-startup-index: 0;
+    // Playback — Qobuz Connect device name. The persisted custom override
+    // ("" = unset) plus the effective default name shown as the input
+    // placeholder ("Qbz - {hostname}"). Backed by the QConnect settings DB
+    // via the settings-string callback; applies on the next connect.
+    in-out property <string> qconnect-device-name: "";
+    in-out property <string> qconnect-device-name-default: "";
 
     // --- OFFLINE MODE --------------------------------------------------------