# M5: Pads tab

Branch: `m5-pads-tab` → `main`

## What changed

**Go service** (`internal/app`), new methods:
- `SetName`, `AssignSampleDialog`, `AssignSamplePath` (used by file drop), `ClearPad`, `SetTune`, `SetMuteGroup`.
- Export: `ChooseExportFolder` / `ExportFolder(dir, overwrite)` and `ChooseZipPath` / `ExportZip(path, overwrite)`.
  - If the target exists, nothing is written and `exists` lists it; the frontend asks, then calls again with overwrite.
  - A zip path without `.zip` gets it added.
  - A successful export clears the unsaved-changes flag.
- Every edit returns the new state, even when it fails. The frontend refetches the state when a call is rejected.
- `internal/app` still imports nothing from Wails, so its tests run anywhere. The native dialogs are behind a small `Dialogs` interface; `dialogs.go` implements it with Wails.

**Preview audio** (`internal/app/audio.go`, `frontend/src/lib/audio.ts`), build plan 4.5:
- Go serves each pad's source WAV at `/audio/<slot>?k=…` through the asset server's middleware. The key changes with the source file, so the preview never plays a stale cached sample.
- The frontend decodes with Web Audio. If the webview can't decode a format, it fetches `/audio/<slot>?pcm=1`, which Go decodes (the M3 decoder).
- Tuning is applied as detune (coarse × 100 + fine cents), so pitch and speed change together as on the device. Only the slice region is played.
- Pads are monophonic: a new click restarts the pad. Starting a pad cuts off every sounding pad in the same non-zero mute group with an 8 ms fade.

**File drop** (`main.go`): the window accepts files. Pad cells and the empty-pad panel are drop targets carrying `data-slot`, and they highlight while a file is over them. Go assigns the first `.wav` dropped and sends the new state as a `state` event; problems come as an `error` event. Dropping a non-WAV file shows a message.

**Frontend:**
- Project name is editable (Enter or leaving the field saves; a refused name goes back).
- Export zip and Export to folder work, with a confirm dialog before replacing files.
- Clicking a pad selects it and plays it.
- The pad panel is live: Choose WAV file, Play, Replace, Clear, tuning sliders (they play the pad when let go), mute group (Off, 1–16).
- Errors show in a dismissable banner. The footer shows "Unsaved changes" and where the last export went.
- Move/Copy stay disabled until M6. Load stays disabled until M8.

**`tools/xpjdiff`**: `go run ./tools/xpjdiff A.xpj B.xpj` compares two projects (the .xpj values and the data-folder files) and prints the differing paths, or `identical`. It's for checking this milestone.

## Testing

On the server:
- `go vet` and `go test ./...` pass (Linux with `-tags purego`, and `GOOS=windows go vet`).
- The service tests cover the dialog-cancel path, assign/clear/tune/mute through the service, name validation, folder and zip export with the overwrite round-trip, an export through the service that matches the model byte for byte, and the audio route (WAV bytes, decoded PCM header and peak, 404s, other paths passed through).
- `xpjdiff` is tested against T1/T2/T3.

The server has no Node, so the frontend is untested here. **On the laptop**, from the repo root (create `C:\temp\m5` first). Use quotes as shown; in PowerShell an unquoted comma splits an argument.

1. `npm run check` in `frontend`: 0 errors, 0 warnings.
2. `wails3 dev`. In the app:
   - Name the project `UiKit`.
   - A01: **Choose WAV file** → `testdata\device\DemoFile_[ProjectData]\PadLp-London Cm114 SA.wav`.
   - A02: **drag** `testdata\device\DemoFile_[ProjectData]\Bass-Pure C.wav` from Explorer onto the A02 cell.
   - A02: Semitones −12, Cents +50.
   - A01 and A02: mute group 1.
   - **Export to folder** → `C:\temp\m5`.
3. The same edits with the CLI:
   ```
   go run ./cmd/mpckit-cli -name CliKit -out C:\temp\m5 -pad "A1=testdata\device\DemoFile_[ProjectData]\PadLp-London Cm114 SA.wav" -pad "A2=testdata\device\DemoFile_[ProjectData]\Bass-Pure C.wav" -tune "A2=-12,50" -mute "A1=1" -mute "A2=1"
   ```
4. `go run ./tools/xpjdiff C:\temp\m5\UiKit.xpj C:\temp\m5\CliKit.xpj` prints `identical`. **(M5 done-when, part 1.)**
5. Preview: click A01 (a long pad sound), then A02 while it plays: A01 stops. Set A02's group to Off and repeat: both play together. A02 sounds 11.5 semitones lower than untuned (just under an octave). **(M5 done-when, part 2.)**
6. Export again to the same folder: a dialog asks before replacing; Cancel writes nothing. Export zip saves `UiKit.zip`.
7. Clear A02: the pad empties and the footer drops to 1 sample. Rename to `a/b`: an error shows and the name goes back.