feat(settings): Open Source Licenses section in System tab (#477)

* feat(licenses): tooling and initial data generation

Adds the maintainer-only generator that produces src/data/licenses.json:
- src-tauri/about.toml + about.hbs: cargo-about config + handlebars template
  for the Rust-side license enumeration
- scripts/generate-licenses.mjs: orchestrator that runs cargo-about and
  license-checker-rseidelsohn (via npx, no devDep), merges the outputs into
  a single per-crate JSON with full license texts, and writes the result to
  src/data/licenses.json

The script is invoked directly with `node scripts/generate-licenses.mjs` —
no npm script wrapper on purpose, since adding one to package.json would
trigger the nix-npm-deps-hash-sync workflow on every push.

Initial generation covers 575 cargo crates + 71 npm packages (646 entries
total, all with full license text bundled, ~1.4 MB JSON).

* feat(licenses): Settings panel UI

Adds a new Open Source Licenses section under Settings → System,
sitting below Contributors. Components:

- LicensesPanel.tsx: search input, curated highlight block of ~10 key
  dependencies (Tauri, React, rodio, symphonia, etc.), TanStack-Virtual
  list of all 600+ entries
- LicenseTextModal.tsx: full-screen-ish modal showing the bundled license
  text plus name/version/license-id badges + repository link
- licensesData.ts: lazy dynamic-import loader (Vite emits the JSON as a
  separate chunk, so the heavy ~1.4 MB payload is only loaded when the
  user actually opens the panel — no runtime fetch, the data is fixed
  into the build artifact)

The panel registers itself in the Settings in-page search index under
the System tab.

* feat(licenses): i18n in 8 locales

Adds the `licenses` namespace (title, intro, highlights, search
placeholder, no-results, loading / load error, no-license-text,
view-source, total line, generated-at) across en, de, fr, nl, zh, nb,
ru, es. License names themselves (MIT, Apache-2.0, GPL-3.0, …) stay
universal and are rendered as-is.

* docs(release): document licenses regeneration step

Adds a Step A.3 to the release SOP describing the maintainer-only
`node scripts/generate-licenses.mjs` workflow, the cargo-about
prerequisite, and explicitly notes why no npm script wrapper exists
(would trigger the nix-npm-deps-hash-sync workflow).

src/locales/nl.ts

@@ -1759,4 +1759,17 @@ export const nlTranslation = {
     exitPsysonic: 'Psysonic afsluiten',
     nothingPlaying: 'Niets wordt afgespeeld',
   },
+  licenses: {
+    title: 'Open-source licenties',
+    intro: 'Psysonic is gebouwd op open-source software. De onderstaande lijst toont elke afhankelijkheid die met de app wordt meegeleverd, met versie, licentie en volledige licentietekst.',
+    highlights: 'Belangrijke afhankelijkheden',
+    searchPlaceholder: 'Zoek op naam, versie of licentie…',
+    noResults: 'Geen resultaten.',
+    loading: 'Licenties laden…',
+    loadError: 'Licentiegegevens konden niet worden geladen.',
+    noLicenseText: 'Geen licentietekst meegeleverd voor deze afhankelijkheid.',
+    viewSource: 'Bron bekijken',
+    totalLine: '{{total}} afhankelijkheden — {{cargo}} cargo, {{npm}} npm',
+    generatedAt: 'Laatst gegenereerd {{date}}',
+  },
 };