refactor: remove Tauri auto-updater, replace with simple GitHub-check modal

Remove tauri-plugin-updater, relaunch_after_update command, generate-update-
manifest.js, the generate-manifest CI job, and all signing/upload steps for
.sig files and latest.json. The updater plugin, its pubkey and endpoint config
in tauri.conf.json, and the updater:default capability are all gone.

Replace AppUpdater.tsx with a lightweight component that:
- Fetches https://api.github.com/repos/Psychotoxical/psysonic/releases/latest
  after a 4 s delay (no install, no download, no progress bar)
- Shows a dismissible toast with two buttons:
  GitHub → releases/latest
  Website → https://psysonic.psychotoxic.eu/#downloads

i18n: remove updaterInstall/Downloading/Installing/Download/ExperimentalHint
keys from all 7 locales; add updaterWebsite; update updaterVersion wording.

CI: remove generate-manifest job and all .sig signing/upload steps from
build-macos-windows. Also remove TAURI_SIGNING_PRIVATE_KEY env vars (no
longer needed). Release now just builds and uploads the installable binaries.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/locales/fr.ts

@@ -15,9 +15,6 @@ export const frTranslation = {
     help: 'Aide',
     expand: 'Développer la barre latérale',
     collapse: 'Réduire la barre latérale',
-    updateAvailable: 'Mise à jour disponible',
-    updateReady: '{{version}} est prêt',
-    updateLink: 'Voir la version →',
     downloadingTracks: '{{n}} pistes en cache…',
     offlineLibrary: 'Bibliothèque hors ligne',
     genres: 'Genres',
@@ -328,12 +325,8 @@ export const frTranslation = {
     bulkRemoveFromPlaylist: 'Retirer de la playlist',
     bulkClear: 'Désélectionner',
     updaterAvailable: 'Mise à jour disponible',
-    updaterVersion: 'v{{version}} est prête',
-    updaterInstall: 'Installer & Redémarrer',
-    updaterDownloading: 'Téléchargement…',
-    updaterInstalling: 'Installation…',
-    updaterDownload: 'Télécharger depuis GitHub',
-    updaterExperimentalHint: 'La mise à jour automatique est encore en développement',
+    updaterVersion: 'v{{version}} est disponible',
+    updaterWebsite: 'Site Web',
   },
   settings: {
     title: 'Paramètres',