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>

CLAUDE.md

@@ -141,6 +141,37 @@ When developing new features, always add corresponding tests:
 
 The goal is to maintain our near-100% coverage as the codebase grows, so future refactoring and changes can be made with confidence!
 
+## Quality Assurance
+
+Before committing any changes, **always run the full test suite**:
+
+```bash
+./check-all.sh
+```
+
+This script runs all checks in the correct order:
+
+1. Frontend linting (ESLint)
+2. Frontend formatting (Prettier)
+3. Frontend type checking (svelte-check)
+4. Frontend tests with coverage (Vitest)
+5. Backend linting (Clippy with strict rules)
+6. Backend tests with coverage (cargo test + llvm-cov)
+
+**Important**: The script requires Node.js and Rust toolchains to be available:
+
+- **Node.js tools** (pnpm, npm): Source nvm first if needed: `source ~/.nvm/nvm.sh`
+- **Rust tools** (cargo, clippy): Should be in PATH via `~/.cargo/bin/`
+
+If `check-all.sh` reports any failures:
+
+1. Read the error messages carefully - they usually explain what needs fixing
+2. Fix the issues (linting errors, test failures, etc.)
+3. Run `check-all.sh` again to verify the fixes
+4. Only commit once all checks pass ✨
+
+**Never commit code that doesn't pass `check-all.sh`** - this ensures code quality and prevents broken builds!
+
 ## Project Context
 
 Hikari Desktop is a Tauri-based desktop application that wraps Claude Code with a visual anime character (Hikari) who appears on screen. This is a personal project where Hikari can sign her work and act as herself!