Fix prod-readiness issues flagged in audit

- Fix XDG config dir logic in config/mod.rs (was double-nesting and had /home/user hardcode)
- Replace /home/user hardcodes in breadbar.rs and hyprland.rs with config::config_dir()
- Fix /home/user hardcode in packages.rs (uses /root fallback for .local/state path)
- Remove eprintln! from GTK callback in packages.rs (no stderr at runtime)
- Fix YAML parse error in branding.desc (missing space after sidebarTextHighlight key)
- Add .gitignore (Rust target/, ISO artifacts, editor/OS junk, secrets)
- Delete state.rs (dead code — never mod'd in main.rs)
- Add brightnessctl, grim, slurp to packages.x86_64 (used by keybinds)
- Rename can-you-begin-a-composed-beacon.md → DESIGN.md

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

iso/airootfs/etc/calamares/branding/bos/branding.desc

@@ -26,4 +26,4 @@ style:
     sidebarBackground:   "#3b4252"
     sidebarText:         "#eceff4"
     sidebarTextSelect:   "#5e81ac"
-    sidebarTextHighlight:"#eceff4"
+    sidebarTextHighlight: "#eceff4"