Rename lovart /generate* routes to /avatar

Create a proj-doc for walkingweekend.com: Rename lovart /generate* routes to /avatar (type: refactoring). The proj-doc should plan and track the work needed to rename the lovart /generate* route family to /avatar. Concretely: rename /generate, /generate/stream, /generate-batch and /generate-prompts, and update every caller — components/teaching-main.tsx, scripts/canvas-article-image.mjs, the four action docs under content/docs/actions, the shared testing submodule fixtures, and bearlabs.net two lovart proj-docs plus src/data/testing/walkingweekend.json. Decide explicitly whether the old path keeps a back-compat alias or hard-breaks. No SKILL.md references /generate, so skills need no change.