ESP/Speaker: Remove unnecessary heap allocation for I2S channel handle

## Problem

`i2s_chan_handle_t` is itself a pointer type. `init_amp()` currently allocates a heap object solely to provide storage for that pointer:
```cpp
i2s_chan_handle_t *tx_handle = new i2s_chan_handle_t();
i2s_new_channel(..., tx_handle, ...);
```

This allocation is unnecessary. It also makes ownership unclear because the wrapper pointer is allocated in `audio.cpp` and later deleted in `tas5805.cpp`.

An allocation failure during startup may also occur before the code has an opportunity to handle the error cleanly.

## Proposed change

Store the handle directly:
```cpp
i2s_chan_handle_t tx_handle = nullptr;
i2s_new_channel(..., &tx_handle, ...);
```

Store the resulting handle by value in `TAS5805`:
```cpp
i2s_chan_handle_t tx_handle = nullptr;
```
Update callers to use:
```
i2s_channel_write(tx_handle, ...);
```
rather than dereferencing a pointer-to-handle.

Do not delete the handle. If TAS5805 owns the lifetime of the I2S channel, release it with `i2s_del_channel()` when the channel is no longer needed.

Read init_amp() in audio.cpp and the TAS5805 implementation in tas5805.cpp, tracing how tx_handle is created, stored, used, and deleted. Change the handle to a value in both locations, update i2s_channel_write() callers, and verify the channel lifetime is handled with i2s_del_channel() when appropriate; done means the heap wrapper and delete are gone and the project builds.

Written by the indexing model from the issue text.