feat(qbz-qt): phase 19 — full Settings subnav, Appearance + Integrations, live theming

Theme system as data: qbz_theme registry (36 themes + auto/custom)
resolved from the shared ui_prefs slug into a themeJson document;
QbzTheme rebound to it so the whole app repaints live (OLED->Light
probe-verified). Full subnav (8 sections + Share logs footer);
Appearance 1:1 (theme dropdown with dark/light filter, ambient/titlebar
rows in their real home, typography, library visuals, window chrome,
tray via tray_settings.db, renderer); Integrations 1:1 (MusicBrainz,
Last.fm 2-step browser connect + authed state, ListenBrainz token
validate, Discord toggle -> live DiscordRpc.set_enabled). Offline/Local
Library/Blacklist/Developer render honest placeholders.

km

crates/qbz-qt/Cargo.toml

@@ -37,6 +37,8 @@ qbz-text-utils = { path = "../qbz-text-utils" }
 qbz-credentials = { path = "../qbz-credentials" }
 qbz-i18n = { path = "../qbz-i18n" }
 qbz-log = { path = "../qbz-log" }
+qbz-theme = { path = "../qbz-theme" }
+qbz-integrations = { path = "../qbz-integrations" }
 
 rusqlite = { version = "0.31", features = ["bundled"] }
 image = { version = "0.25", default-features = false, features = ["jpeg", "png"] }