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/nl.ts

@@ -15,9 +15,6 @@ export const nlTranslation = {
     help: 'Help',
     expand: 'Zijbalk uitklappen',
     collapse: 'Zijbalk inklappen',
-    updateAvailable: 'Update beschikbaar',
-    updateReady: '{{version}} is klaar',
-    updateLink: 'Naar release →',
     downloadingTracks: '{{n}} nummers worden gecached…',
     offlineLibrary: 'Offline bibliotheek',
     genres: 'Genres',
@@ -328,12 +325,8 @@ export const nlTranslation = {
     bulkRemoveFromPlaylist: 'Verwijderen uit afspeellijst',
     bulkClear: 'Selectie wissen',
     updaterAvailable: 'Update beschikbaar',
-    updaterVersion: 'v{{version}} is klaar',
-    updaterInstall: 'Installeren & Herstarten',
-    updaterDownloading: 'Downloaden…',
-    updaterInstalling: 'Installeren…',
-    updaterDownload: 'Downloaden van GitHub',
-    updaterExperimentalHint: 'Automatisch bijwerken is nog in ontwikkeling',
+    updaterVersion: 'v{{version}} beschikbaar',
+    updaterWebsite: 'Website',
   },
   settings: {
     title: 'Instellingen',