Add in-app changelog viewer and repo CHANGELOG.md

- Added CHANGELOG constant to launcher with version history
- Added 'Changelog' button to header bar (opens formatted dialog)
- Title bar now shows version number (v2.0.0)
- Added CHANGELOG.md to repo root
- Rebuilt .deb with latest sources

CHANGELOG.md

@@ -0,0 +1,15 @@
+# Changelog
+
+## v2.0.0 (2026-05-19)
+
+- Initial release: multi-provider Codex Launcher
+- Translation proxy: Responses API to Chat Completions + Anthropic Messages
+- GTK endpoint manager with 10+ provider presets
+- Codex Default mode (built-in OAuth, zero config)
+- Browser UA injection for Cloudflare-protected providers (OpenCode)
+- Streaming SSE, tool calls, reasoning content support
+- Profile backup/import, model auto-fetch, bulk import
+- Refresh Models in background thread
+- URL normalization to prevent double-path bugs
+- Config backup/restore around sessions
+- .deb installer package