license: adopt MIT for the application + add THIRD_PARTY_LICENSES

Set license = "MIT" (Cargo.toml), add the MIT LICENSE file, and switch the
PKGBUILD to license=('MIT'). Generate a THIRD_PARTY_LICENSES file enumerating
the full dependency-graph manifest plus the canonical text of every referenced
license, with notices for the statically-bundled Opus codec and the embedded
fonts (Iced-Icons, Cantarell/OFL-1.1). Ship both files in the .deb and Arch
packages. Update README License section and stale private-build metadata.

Codex (gpt-5.5) audited the tree and confirmed no copyleft blocks MIT; the
remaining items were asset provenance + binary-distribution notice hygiene,
which THIRD_PARTY_LICENSES addresses.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

packaging/PKGBUILD

@@ -6,7 +6,7 @@ pkgrel=1
 pkgdesc="Decentralized peer-to-peer voice chat (Rust/iroh/PipeWire/Opus/iced)"
 arch=('x86_64')
 url="https://gitbutter.xyz/mollusk/peerspeak"
-license=('custom')
+license=('MIT')
 depends=('pipewire' 'opus')
 makedepends=('git' 'cargo' 'pkgconf')
 optdepends=('pixelpass: screen sharing inside a room'
@@ -70,4 +70,9 @@ package() {
     install -Dm644 "assets/icons/$_pkgname-$s.png" \
       "$pkgdir/usr/share/icons/hicolor/${s}x${s}/apps/$_pkgname.png"
   done
+
+  # License + third-party attribution notices.
+  install -Dm644 LICENSE "$pkgdir/usr/share/licenses/$_pkgname/LICENSE"
+  install -Dm644 THIRD_PARTY_LICENSES \
+    "$pkgdir/usr/share/licenses/$_pkgname/THIRD_PARTY_LICENSES"
 }