Add start.sh for unified WASM build and frontend dev server launch

[Copilot]

Adds ./start.sh as single-command entrypoint to build and run the application.

Implementation

• Prerequisites validation: Checks Node.js, npm/yarn, Rust, wasm-pack with version reporting
• WASM build: Invokes pdf-utils/wasm/generate_wasm.sh if toolchain available, degrades gracefully if not
• Dependency management: Prefers yarn over npm, installs frontend deps automatically
• Dev server: Launches Vite on localhost:3000

Technical Details

• Uses set -euo pipefail for fail-fast execution
• Color-coded log levels (INFO/WARN/ERROR/STEP) for output clarity
• Resolves script directory dynamically for path independence
• Shellcheck compliant (SC2155 addressed via separate declare/assign)

Usage

./start.sh
# Validates prerequisites → builds WASM → installs deps → starts dev server

Script auto-detects missing tools and provides installation guidance rather than failing silently.

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