fix: validate Claude binary installation before connection (#138)

## Summary

Add validation to check that the Claude CLI is installed before attempting to start a connection. If the `claude` binary is not found, users receive a helpful error message with installation instructions.

## Changes

- ✅ Add Claude binary check using `which` command in `WslBridge::start()`
- ✅ Return clear error message with installation command if not found
- ✅ Add test coverage for the binary check logic (`test_claude_binary_check_command_structure`)
- ✅ Update `CLAUDE.md` with Quality Assurance section documenting `check-all.sh`

## Error Message

If Claude Code is not installed, users will see:
```
Claude Code is not installed. Please install it using:

curl -fsSL https://claude.ai/install.sh | bash
```

## Testing

- All 427 backend tests pass ✅
- All 387 frontend tests pass ✅
- `check-all.sh` passes with no errors ✅
- New test validates the `which claude` command structure

## Documentation Updates

Added comprehensive Quality Assurance section to `CLAUDE.md` explaining:
- How to run `check-all.sh` before committing
- What checks are included and their order
- How to source necessary binaries (nvm for Node.js)
- Troubleshooting steps for failures

✨ This pull request was created by Hikari~ 🌸

Co-authored-by: Naomi Carrigan <commits@nhcarrigan.com>
Reviewed-on: #138
Co-authored-by: Hikari <hikari@nhcarrigan.com>
Co-committed-by: Hikari <hikari@nhcarrigan.com>

src-tauri/Cargo.lock

@@ -1636,7 +1636,7 @@ checksum = "7f24254aa9a54b5c858eaee2f5bccdb46aaf0e486a595ed5fd8f86ba55232a70"
 
 [[package]]
 name = "hikari-desktop"
-version = "1.4.0"
+version = "1.5.0"
 dependencies = [
  "chrono",
  "dirs 5.0.1",