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>

scripts/generate-update-manifest.js

@@ -1,90 +0,0 @@
-#!/usr/bin/env node
-// Generates latest.json for the Tauri updater from a GitHub release.
-// Reads .sig files uploaded by tauri-action, assembles the manifest, writes latest.json.
-//
-// Required env vars: VERSION, GITHUB_TOKEN
-// Usage: node scripts/generate-update-manifest.js
-
-const { execSync } = require('child_process');
-const fs = require('fs');
-
-const VERSION = process.env.VERSION;
-const REPO = 'Psychotoxical/psysonic';
-const TAG = `app-v${VERSION}`;
-
-if (!VERSION) {
-  console.error('VERSION env var required');
-  process.exit(1);
-}
-
-// Platform → update bundle filename (produced by tauri-action with updater plugin)
-const PLATFORM_FILES = {
-  'darwin-aarch64': `Psysonic_aarch64.app.tar.gz`,
-  'darwin-x86_64':  `Psysonic_x64.app.tar.gz`,
-  'windows-x86_64': `Psysonic_${VERSION}_x64-setup.nsis.zip`,
-};
-
-const platforms = {};
-
-// A real minisign .sig file is multi-line and ~200+ chars.
-// A public key (RWTxxx... single line, ~56 chars) must never appear here.
-function validateSignature(sig, platform, sigFile) {
-  // Public keys start with RWT and are a single short base64 token
-  if (/^RWT[A-Za-z0-9+/]{10,}={0,2}$/.test(sig)) {
-    throw new Error(
-      `${platform}: .sig file "${sigFile}" contains a PUBLIC KEY instead of a signature.\n` +
-      `  Got: ${sig}\n` +
-      `  The TAURI_SIGNING_PUBLIC_KEY env var must never be used as the signature value.\n` +
-      `  Ensure the signing step correctly writes the .sig file from the private key.`
-    );
-  }
-  if (sig.length < 80) {
-    throw new Error(
-      `${platform}: .sig file "${sigFile}" looks too short (${sig.length} chars) to be a valid signature.`
-    );
-  }
-}
-
-for (const [platform, filename] of Object.entries(PLATFORM_FILES)) {
-  const sigFile = `${filename}.sig`;
-  try {
-    execSync(
-      `gh release download "${TAG}" --repo "${REPO}" -p "${sigFile}" --clobber`,
-      { stdio: 'pipe' }
-    );
-    const signature = fs.readFileSync(sigFile, 'utf8').trim();
-    validateSignature(signature, platform, sigFile);
-    const url = `https://github.com/${REPO}/releases/download/${TAG}/${filename}`;
-    platforms[platform] = { signature, url };
-    console.log(`✓ ${platform}`);
-  } catch (e) {
-    console.warn(`⚠ Skipping ${platform}: ${e.message}`);
-  }
-}
-
-if (Object.keys(platforms).length === 0) {
-  console.error('No platforms found — aborting manifest generation');
-  process.exit(1);
-}
-
-// Pull release notes from GitHub
-let notes = '';
-try {
-  const raw = execSync(
-    `gh release view "${TAG}" --repo "${REPO}" --json body`,
-    { stdio: 'pipe' }
-  ).toString();
-  notes = JSON.parse(raw).body ?? '';
-} catch {
-  console.warn('Could not fetch release notes');
-}
-
-const manifest = {
-  version: VERSION,
-  notes,
-  pub_date: new Date().toISOString(),
-  platforms,
-};
-
-fs.writeFileSync('latest.json', JSON.stringify(manifest, null, 2));
-console.log(`\nWrote latest.json for v${VERSION} with platforms: ${Object.keys(platforms).join(', ')}`);