Rename project from mpv-mgr to empeve

Rebrand the entire project with a memorable phonetic name:
- Update package and binary name in Cargo.toml
- Change config directory from ~/.config/mpv-mgr to ~/.config/empeve
- Rename MpvMgrError to EmpveError
- Update all CLI help text and user-facing messages
- Update README title and documentation

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

src/error.rs

@@ -1,7 +1,7 @@
 use thiserror::Error;
 
 #[derive(Error, Debug)]
-pub enum MpvMgrError {
+pub enum EmpveError {
     #[error("Config error: {0}")]
     Config(String),
 
@@ -30,4 +30,4 @@ pub enum MpvMgrError {
     InvalidRepo(String),
 }
 
-pub type Result<T> = std::result::Result<T, MpvMgrError>;
+pub type Result<T> = std::result::Result<T, EmpveError>;