feat: add separate instances_dir config option

[OneNoted]

Summary

• Adds a new instances_dir config option that lets users override the instances path independently of data_dir
• Useful when instances live on a separate drive or non-standard location
• Falls back to <data_dir>/instances when unset; validates the path exists on startup
• Documents the new option in the README

Test plan

• Run without either option set — auto-detection unchanged
• Set only instances_dir — uses auto-detected data_dir but custom instances path
• Set both data_dir and instances_dir — each respected independently
• Set instances_dir to nonexistent path — app shows error
• cargo clippy -- -D warnings passes
• cargo test passes (15/15)

Greptile Summary

Added a new instances_dir config option that allows users to specify a custom location for Minecraft instances independent of the main PrismLauncher data_dir. This is useful when instances are stored on a separate drive or non-standard location.

Key changes:

• Added instances_dir field to AppConfig with tilde expansion support
• Modified PrismConfig::load() to accept and validate the instances directory override
• Updated instances_dir() method to return the override path when set, otherwise falls back to <data_dir>/instances
• Validated that the instances directory exists during config load, returning an error if not
• Documented the new option in README with clear examples

Confidence Score: 5/5

• This PR is safe to merge with minimal risk
• Clean implementation following existing patterns, proper validation added, well-documented, and author reports tests pass
• No files require special attention

Important Files Changed

Filename	Overview
README.md	documented new instances_dir config option with clear explanations and examples
src/data/app_config.rs	added instances_dir field with tilde expansion support matching data_dir pattern
src/data/config.rs	implemented instances_dir override with validation; stylistic preference for separate if statement instead of let-chain
src/main.rs	wired up instances_dir config parameter to PrismConfig::load()

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
    Start[App Start] --> LoadAppConfig[Load AppConfig from config.toml]
    LoadAppConfig --> CheckDataDir{data_dir set?}
    CheckDataDir -->|Yes| ValidateDataDir[Validate data_dir exists]
    CheckDataDir -->|No| AutoDetect[Auto-detect PrismLauncher data_dir]
    ValidateDataDir --> DataDirOK[data_dir resolved]
    AutoDetect --> DataDirOK
    DataDirOK --> LoadPrismConfig[Load PrismConfig]
    LoadPrismConfig --> CheckInstancesDir{instances_dir set?}
    CheckInstancesDir -->|Yes| ValidateInstancesDir[Validate instances_dir exists]
    CheckInstancesDir -->|No| UseDefault[Use <data_dir>/instances]
    ValidateInstancesDir -->|Exists| InstancesDirOK[instances_dir resolved]
    ValidateInstancesDir -->|Not Found| Error1[Error: instances_dir not found]
    UseDefault --> InstancesDirOK
    InstancesDirOK --> AppReady[App Ready]

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_{Last reviewed commit: 6ef7df4}

[greptile-apps]

_{4 files reviewed, 1 comment}

_{Edit Code Review Agent Settings | Greptile}

[greptile-apps]

validation happens before data_dir is validated in main.rs - if data_dir validation fails first, this error won't be shown

Suggested change

	if let Some(ref dir) = instances_dir_override
	&& !dir.exists()
	if let Some(ref dir) = instances_dir_override {
	if !dir.exists() {

Prompt To Fix With AI

This is a comment left during a code review.
Path: src/data/config.rs
Line: 15-16

Comment:
validation happens before `data_dir` is validated in `main.rs` - if `data_dir` validation fails first, this error won't be shown

```suggestion
        if let Some(ref dir) = instances_dir_override {
            if !dir.exists() {
```

How can I resolve this? If you propose a fix, please make it concise.