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>

deny.toml

@@ -81,8 +81,8 @@ allow = [
 confidence-threshold = 0.8
 exceptions = []
 
-# peerspeak itself has no `license` field and is not published, so skip the
-# "unlicensed" check for our own (private) crate. Add a license to Cargo.toml
-# if/when this is ever published.
+# peerspeak is MIT-licensed (see Cargo.toml `license` + the LICENSE file) but is
+# not published to crates.io, so keep the private-crate skip for the
+# "unlicensed"/publish checks. MIT is already in the allow list above.
 [licenses.private]
 ignore = true