docs: update AGENTS.md and README for new features, add JS tests

Docs:
- AGENTS.md: updated toolchain section with js-lint/js-test tasks,
  expanded web/ui/ directory listing, added event delegation and
  Web Audio API to key patterns, updated testing section
- README: marked issue #9 (Audio Preview Player) as done
- .gitignore: added akai-fetch.test and cover.out

Tests:
- Moved groupWavs and cleanName to util.js (pure functions)
- Added 10 new JS tests: 6 groupWavs, 4 cleanName
- Total JS tests: 29 (up from 19)

AGENTS.md

@@ -16,7 +16,9 @@ mise run serve             # → starts HTTP server, opens browser
 mise run electron-deps     # installs electron + electron-builder
 mise run electron-dev      # runs electron in dev mode
 mise run docker-up         # docker compose up --build
-mise run lint              # go vet ./...
+mise run lint              # golangci-lint run (Go)
+mise run js-lint            # eslint (JS)
+mise run js-test            # node --test (JS)
 mise run check-all         # verify gcc, brew, mise, go, node, docker, colima
 ```
 
@@ -55,7 +57,14 @@ Single binary (`fetch`) with subcommands. No external Go dependencies — pure s
 ```
 main.go        CLI entry, commands (search/list/download/extract/pipeline/serve)
 serve.go       HTTP server + REST API + embedded web UI (embed.FS)
-web/ui/        Static frontend (index.html, style.css, app.js)
+web/ui/        Static frontend
+  ├── index.html              HTML layout
+  ├── style.css               CSS custom properties theme
+  ├── util.js                 shared utility functions
+  ├── app.js                  application logic, event delegation
+  ├── util.test.js            JS tests (node --test)
+  ├── eslint.config.js        ESLint flat config
+  └── package.json            ESLint + test scripts
 electron/      Electron wrapper shell
 scripts/       Bash helpers called via exec
   ├── extract_wavs.sh            batch WAV extraction via akaiutil
@@ -70,8 +79,9 @@ third_party/   Vendored akaiutil C binary (GPLv2)
 - **Flag parsing**: each command creates its own `flag.NewFlagSet` — avoids global flag state
 - **Concurrency**: semaphore channel pattern (`sem := make(chan struct{}, N)`) for bounded parallelism
 - **SSE progress**: `serve.go:handleProgress` — 500ms ticker, EventSource stream, auto-closes when all done
+- **Event delegation**: `app.js` — single click listener on document, dispatches by `[data-action]` attribute (download, list, play)
 - **akaiutil wrapper**: `extract.go` — persistent stdin/stdout pipes, no bash intermediary
-- **Zero-deps web UI**: vanilla JS, no bundler, no framework. API calls use `fetch()` + `EventSource`
+- **Zero-deps web UI**: vanilla JS, no bundler, no framework. API calls use `fetch()` + `EventSource`. Web Audio API for WAV playback.
 - **Embed directive**: `//go:embed web/ui/*` in `serve.go` — frontend baked into the binary
 - **Bind address handling**: `serve.go:37` — `--host` flag defaults to `127.0.0.1` for local dev; set `--host 0.0.0.0` for Docker. When binding to `0.0.0.0`, prints both `localhost` and `<host-ip>` URLs for clarity.
 
@@ -124,12 +134,15 @@ All akaiutil calls use `-r` (read-only) flag. Write operations (S900 compress, p
 
 ## Testing
 
-No test framework yet. Manual verification:
 ```bash
-go build -o /dev/null .        # compile check
+# Go
+go test -cover                  # unit tests (extract, serve, http, download, cli, util)
 go vet ./...                   # static analysis
-golangci-lint run              # comprehensive lint
-./akai-fetch serve              # manual smoke test
+golangci-lint run              # comprehensive lint (uses .golangci.yml)
+
+# JavaScript
+mise run js-test               # node --test (19 tests: esc, sanitize, formatBytes, formatTime)
+mise run js-lint               # eslint (flat config)
 ```
 
 ## Secrets & Tokens