feat: bootstrap Tauri 2 + React/TypeScript foundation (issue #14)

[thundermiracle]

Summary

• scaffold Tauri 2 + React/TypeScript app foundation
• add frontend app shell (src/) and Tauri backend shell (src-tauri/)
• configure project scripts for desktop dev/build (pnpm tauri:dev, pnpm tauri:build)
• update Rust crate to edition = "2024"
• add bootstrap validation tests (tests/tauri-bootstrap.test.mjs)
• add icon generation script for clean environments (scripts/ensure-tauri-icon.mjs)
• update AGENTS.md with:
  • latest stable dependency selection policy
  • mandatory pre-commit lint/test checks
  • Rust/TypeScript split-commit rule

Follow-up Fixes in this PR

• fix src-tauri/src/lib.rs cfg condition and then remove mobile entry attribute for macOS-only scope
• make lint:rust self-sufficient by running ensure:tauri-icon first

Validation

• pnpm run lint (pass)
• pnpm test (23/23 pass)
• pnpm build (pass)
• pnpm tauri:build (pass)
• pnpm tauri:dev startup verified

Commit Structure

• Rust-only commits for Rust entrypoint/lint scope adjustments
• TypeScript/workflow commits for scripts/tests/docs/agent rules

Closes #14