Combine atomic file operations with exclusive locking

This commit combines atomic file operations with exclusive file locking
to ensure safe concurrent access to configuration files.

Changes:
- Keep atomic file operations (write to temp file, then rename)
- Add exclusive locking on config file during write operations
- Lock is acquired in load_mut_config_db before reading
- Lock is held throughout the read-modify-write cycle
- Lock is automatically released when JuliaupConfigFile is dropped
- Removes separate lock file mechanism

The approach provides two layers of safety:
1. Exclusive locking ensures write operations are sequenced
2. Atomic rename ensures readers never see partial writes

This eliminates race conditions while keeping the code simple.

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

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

Author: Keno

src/bin/julialauncher.rs

@@ -316,7 +316,7 @@ fn run_app() -> Result<i32> {
     do_initial_setup(&paths.juliaupconfig)
         .with_context(|| "The Julia launcher failed to run the initial setup steps.")?;
 
-    let config_file = load_config_db(&paths, None)
+    let config_file = load_config_db(&paths)
         .with_context(|| "The Julia launcher failed to load a configuration file.")?;
 
     let versiondb_data = load_versions_db(&paths)

src/command_api.rs

@@ -39,7 +39,7 @@ pub fn run_command_api(command: &str, paths: &GlobalPaths) -> Result<()> {
         other_versions: Vec::new(),
     };
 
-    let config_file = load_config_db(paths, None).with_context(|| {
+    let config_file = load_config_db(paths).with_context(|| {
         "Failed to load configuration file while running the getconfig1 API command."
     })?;
 

src/command_config_backgroundselfupdate.rs

@@ -57,7 +57,7 @@ pub fn run_command_config_backgroundselfupdate(
             }
         }
         None => {
-            let config_file = load_config_db(paths, None)
+            let config_file = load_config_db(paths)
                 .with_context(|| "`config` command failed to load configuration data.")?;
 
             if !quiet {

src/command_config_modifypath.rs

@@ -41,7 +41,7 @@ pub fn run_command_config_modifypath(
             }
         }
         None => {
-            let config_file = load_config_db(paths, None)
+            let config_file = load_config_db(paths)
                 .with_context(|| "`config` command failed to load configuration data.")?;
 
             if !quiet {

src/command_config_startupselfupdate.rs

@@ -47,7 +47,7 @@ pub fn run_command_config_startupselfupdate(
             }
         }
         None => {
-            let config_file = load_config_db(paths, None)
+            let config_file = load_config_db(paths)
                 .with_context(|| "`config` command failed to load configuration data.")?;
 
             if !quiet {

src/command_config_symlinks.rs

@@ -43,7 +43,7 @@ pub fn run_command_config_symlinks(
             }
         }
         None => {
-            let config_file = load_config_db(paths, None)
+            let config_file = load_config_db(paths)
                 .with_context(|| "`config` command failed to load configuration data.")?;
 
             if !quiet {

src/command_config_versionsdbupdate.rs

@@ -38,7 +38,7 @@ pub fn run_command_config_versionsdbupdate(
             }
         }
         None => {
-            let config_file = load_config_db(paths, None)
+            let config_file = load_config_db(paths)
                 .with_context(|| "`config` command failed to load configuration data.")?;
 
             if !quiet {

src/command_info.rs

@@ -10,7 +10,7 @@ use anyhow::{bail, Context, Result};
 
 pub fn run_command_info(paths: &GlobalPaths) -> Result<()> {
     #[cfg(feature = "selfupdate")]
-    let config_file = load_config_db(paths, None).with_context(|| {
+    let config_file = load_config_db(paths).with_context(|| {
         "`run_command_update_version_db` command failed to load configuration db."
     })?;
 
@@ -21,7 +21,7 @@ pub fn run_command_info(paths: &GlobalPaths) -> Result<()> {
     };
 
     #[cfg(not(feature = "selfupdate"))]
-    let _config_file = load_config_db(paths, None).with_context(|| {
+    let _config_file = load_config_db(paths).with_context(|| {
         "`run_command_update_version_db` command failed to load configuration db."
     })?;
 

src/command_override.rs

@@ -24,7 +24,7 @@ struct OverrideRow {
 }
 
 pub fn run_command_override_status(paths: &GlobalPaths) -> Result<()> {
-    let config_file = load_config_db(paths, None)
+    let config_file = load_config_db(paths)
         .with_context(|| "`override status` command failed to load configuration file.")?;
 
     let rows_in_table: Vec<_> = config_file

src/command_status.rs

@@ -26,7 +26,7 @@ struct ChannelRow {
 }
 
 pub fn run_command_status(paths: &GlobalPaths) -> Result<()> {
-    let config_file = load_config_db(paths, None)
+    let config_file = load_config_db(paths)
         .with_context(|| "`status` command failed to load configuration file.")?;
 
     let versiondb_data =