chore(wailsv3): v3 conformance pass over project files

Fixes the wails3 dev boot failure and removes remaining v2-isms so the tree matches a stock v3 project.

- frontend/package.json: add missing build:dev script (vite build --minify false --mode development). The dev taskflow runs npm run build:dev, which v2-era package.json never had - this was the wails3 dev boot failure.
- frontend/vite.config.ts: add the @wailsio/runtime vite plugin (typed custom events, as in every v3 template) and the server host/port/strictPort block (5173, matching the dev task).
- Remove stale v2 build dirs build/darwin + build/windows (v2 wails scaffolding; v3 generates its own via build-assets if ever needed).
- Delete untracked frontend/package.json.md5 (v2 change-detection artifact; v3 Taskfile uses content-hash sources) and drop its gitignore rule.
- go.mod: delete the leftover commented-out v2 replace directive.
- README.md: dev/build docs updated to make dev / make build (wails3), dropping the v2 :34115 browser-server paragraph; install link points at v3 docs.

Verified: production vite build green with the plugin, and wails3 dev boots end-to-end (bindings regen, build:dev, vite on :5173, asset server connected, AniTrack 1.6.8 window mapped).

.gitignore

@@ -26,7 +26,6 @@ build/bin
 .task/
 node_modules
 frontend/dist
-package.json.md5
 # Root-level lockfile only: the frontend lockfile IS tracked so CI installs
 # are reproducible and setup-node's npm cache has something to hash.
 /package-lock.json