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-tauri/Cargo.toml

@@ -39,7 +39,6 @@ tokio = { version = "1", features = ["rt", "time"] }
 biquad = "0.4"
 ringbuf = "0.3"
 tauri-plugin-window-state = "2.4.1"
-tauri-plugin-updater = "2"
 tauri-plugin-process = "2"
 souvlaki = { version = "0.8", default-features = false, features = ["use_zbus"] }
 discord-rich-presence = "0.2"