Rename project: battlenet-manager → umutray

Binary, crate, clap app name, ksni tray id, HTTP user-agent, systemd unit,
XDG config dir (co.aleshym/umutray), README, and all log prefixes.

Config path changes from ~/.config/battlenet-manager/ to ~/.config/umutray/.
Existing users should `mv` the old directory if they've customised it;
otherwise defaults get rewritten on next run.

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

Cargo.toml

@@ -1,11 +1,11 @@
 [package]
-name = "battlenet-manager"
+name = "umutray"
 version = "0.1.0"
 edition = "2021"
 description = "Battle.net launcher manager for Linux via umu/Proton-GE"
 
 [[bin]]
-name = "battlenet-manager"
+name = "umutray"
 path = "src/main.rs"
 
 [dependencies]