Add GUI desktop application

- Add gui/ package: CustomTkinter app with dashboard, mods, tools, logs,
  and settings views; first-run SetupWizard for config.json generation
- Add gui.py root entry point (calls gui.run_app())
- Add selection.json for GUI selection state persistence
- Add customtkinter to requirements.txt
- Fix link_mods.py minor issues surfaced during GUI integration
- Add modlist_html/Test_Preset_A.html and Test_Preset_B.html as example inputs
- Update README.md: add GUI prerequisites, gui.py script section, gui/ folder
  structure, customtkinter to prerequisites table
- Update CLAUDE.md: add python gui.py to common commands, document GUI package
  architecture and views

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

CLAUDE.md

@@ -20,6 +20,9 @@ python run.py --skip-fetch --skip-link
 # Diagnose mod folder name / steam_id issues
 python check_names.py
 python check_names.py --fix --fix-ids
+
+# Launch the GUI
+python gui.py
 ```
 
 There is no build step, linter config, or package install beyond `pip install -r requirements.txt`.
@@ -80,6 +83,26 @@ Pass 2 builds `ok_disk_names` — the set of disk names that already match the s
 
 `--fix-ids` corrects `meta.cpp` using steam IDs from `comparison.json` (sourced from Steam Workshop URLs in the HTML presets) as the authoritative source.
 
+### GUI package
+
+`gui/` is a CustomTkinter desktop application wrapping the CLI toolchain. Entry point is `gui.py` at the project root, which calls `gui.run_app()`.
+
+**Key files:**
+- `gui/__init__.py` — sets dark theme + blue color scheme; exports `run_app()`
+- `gui/app.py` — `ArmaModManagerApp` main window; manages view routing, config loading, thread-safe log queue, and background pipeline execution
+- `gui/wizard.py` — `SetupWizard` dialog shown on first launch when no `config.json` exists
+- `gui/_constants.py` — window dimensions, status color constants, file paths
+- `gui/_io.py` — `_QueueWriter` redirects stdout/stderr to a thread-safe queue so pipeline output streams into the Logs view
+
+**Views** (`gui/views/`): each inherits `BaseView`; `build()` runs once on creation, `refresh()` runs on each navigation:
+- `dashboard.py` — overview, status, quick stats
+- `mods.py` — browse and manage downloaded mods by group
+- `tools.py` — link/unlink, rename folders, sync missing mods, check server
+- `logs.py` — real-time log viewer fed from the stdout/stderr queue
+- `settings.py` — in-app editor for `config.json` (server URL, paths, credentials)
+
+**`selection.json`** — GUI selection state file, tracked in git. Persists which mods/groups are selected between GUI sessions. Written by the GUI; safe to delete (GUI recreates it on next save).
+
 ## Python Version Compatibility
 
 Minimum is Python **3.9**. All files that use `X | Y` union type annotations **must** have `from __future__ import annotations` as the first import. Without it, the `|` syntax raises `TypeError` at runtime on Python < 3.10. Every module in `arma_modlist_tools/` already has it; any new CLI script you add must include it too.