Release v0.7.0

chore(release): bump version to v0.6.0

- package.json: version 0.5.0 -> 0.6.0
- src-tauri/tauri.conf.json: version 0.5.0 -> 0.6.0
- src-tauri/Cargo.toml: version 0.5.0 -> 0.6.0
- src-tauri/Cargo.lock: sync anvil lock entry to 0.6.0
- CHANGELOG.md: add [0.6.0] release notes

chore(release): bump version to v0.5.0

- package.json: version 0.4.0 -> 0.5.0
- src-tauri/tauri.conf.json: version 0.4.0 -> 0.5.0
- src-tauri/Cargo.toml: version 0.4.0 -> 0.5.0
- CHANGELOG.md: add [0.5.0] release notes

fix(build): prod ships an empty catalog instead of failing when none …

…is set (#48)

- tools/fetch-catalog.mjs: in prod a missing or *leetcode* ANVIL_CATALOG_URL now degrades to shipping an empty catalog with a ::warning:: (bring-your-own) rather than failing; dev still errors, and a valid non-*leetcode* URL still fetches and bundles
- .env.example: document that the prod catalog fetch is optional (ships empty) rather than fail-closed

chore(release): bump version to v0.3.0

- package.json: version 0.2.0 -> 0.3.0
- src-tauri/tauri.conf.json: version 0.2.0 -> 0.3.0
- src-tauri/Cargo.toml: version 0.2.0 -> 0.3.0
- CHANGELOG.md: add [0.3.0] release notes

chore(release): bump version to v0.2.0

- package.json: version 0.1.0 -> 0.2.0
- src-tauri/tauri.conf.json: version 0.1.0 -> 0.2.0
- src-tauri/Cargo.toml: version 0.1.0 -> 0.2.0