Update CHANGELOG and add export presets and icon files

- Expanded CHANGELOG with project scaffolding and core tech details.
- Added export presets configuration for Windows and Linux.
- Introduced placeholder icon.svg for project branding.
- Corrected application resource modification setting in export presets.

CHANGELOG.md

@@ -1,3 +1,30 @@
 # Chemo-Sim — A First-Person Chemo Treatment Simulator
 
 Changelog for Chemo-Sim. See [README.md](README.md) for game overview and [docs/DEVELOPMENT_ROADMAP.md](docs/DEVELOPMENT_ROADMAP.md) for development plans.
+
+The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.1.0/),
+and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
+
+## [Unreleased]
+
+### Added — Sprint 1: Project Scaffolding & Core Tech
+
+- **1.1** Initialized Godot 4.x project: `project.godot` (name `chemo-sim`,
+  version `0.1.0`, Forward+ renderer with mobile fallback, 1920×1080 window).
+- **1.2** Created directory structure per `docs/ARCHITECTURE.md`
+  (`scenes/`, `scripts/`, `assets/`, `export_templates/`, `build/`), with
+  `.gitkeep` placeholders so empty dirs are tracked.
+- **1.3** Set up Gitea Actions CI/CD (`.gitea/workflows/build.yml`): builds on
+  push/PR to `main`, exports Windows `.exe` + Linux `.x86_64` via the
+  `barichello/godot-ci:4.3` container, uploads artifacts per run.
+  Removed the GitHub Actions workflow in favor of Gitea-only CI.
+- **1.4** Configured export presets (`export_presets.cfg`) for Windows Desktop
+  and Linux (x86_64). Verified both export locally via
+  `godot --export-release` (Windows .exe ~80 MB, Linux .x86_64 ~63 MB).
+- Added placeholder `icon.svg` referenced by `project.godot`.
+
+### Fixed
+
+- Corrected `.gitignore`: removed `*.godot` rule that would have ignored the
+  required `project.godot`; added Godot 4 cache (`.godot/`), local toolchain
+  (`tools/`), and build output ignores.