chore(release): bump to 0.5.0 + add CHANGELOG

Version bump 0.4.0 -> 0.5.0 (Cargo.toml/lock, Windows .iss, PKGBUILD
placeholder) and a new CHANGELOG.md covering the 21 commits since v0.4.0
(A9 context menu, W21 selectable text, A25 clock-skew, A23 screen-share
per-app audio, A15 reliable critical commands, A24 per-peer volume,
Tier-C security caps, Debian/Arch packaging).

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

Cargo.lock

@@ -4871,7 +4871,7 @@ checksum = "35fb2e5f958ec131621fdd531e9fc186ed768cbe395337403ae56c17a74c68ec"
 
 [[package]]
 name = "peerspeak"
-version = "0.4.0"
+version = "0.5.0"
 dependencies = [
  "anyhow",
  "async-trait",