Change UI thread interval from 0ms minimum to 16ms (reduces UI CPU).

Prior to this change, the UI thread was consuming approx. 10-14% CPU
in the idle state, and appeared to be running in an endless loop.
After this change, the UI thread is measured at about 0.1% CPU. I'm
not sure what the current consumption impact is, but it's gotta be
a power saving, even if slim?

Following manual testing with the device after flashing this change,
the UI responsiveness and music playback does not appear to be
negatively affected. The UI actions that were tested were:

- Scrolling through items on the root menu
- Scrolling through settings
- Scrolling through All Tracks with long-press on the bottom of
  the scroll wheel (Page Down) on a device with ~20k songs
- Repeatedly hitting 'next track'
- Forcing a DB update, and scrolling through All Tracks while
  updating the database.

The device was configured with the following settings when tested:
- TTS enabled
- Wired headphones
- Bluetooth enabled but not connected
- No DB auto-update
- Playing music

Prior to this change, the UI thread was consuming approx. 10-14% CPU
in the idle state, and appeared to be running in an endless loop.
On a stock (v1.2.2) build, `tasks` reported:

```
 → tasks
name            core    free stack      run time
IDLE1           1       1.0 KiB         99.6%
IDLE0           0       1.1 KiB         86.5%
ui              any     10.1 KiB        13.7%
console_repl    1       5.6 KiB         0.5%
Tmr Svc         any     1.4 KiB         0.0%
audio_dec       any     19.4 KiB        0.0%
main            0       2.0 KiB         0.0%
lvglDraw        any     2.9 KiB         0.0%
btController    0       2.1 KiB         0.0%
esp_timer       0       3.4 KiB         0.0%
BTC_TASK        1       2.6 KiB         0.0%
worker_1        any     30.1 KiB        0.0%
worker_3        any     5.7 KiB         0.0%
worker_2        any     29.5 KiB        0.0%
worker_0        any     25.7 KiB        0.0%
BTU_TASK        1       3.2 KiB         0.0%
hciT            1       1.5 KiB         0.0%
ipc0            0       1.0 KiB         0.0%
ipc1            1       1.0 KiB         0.0%
audio_conv      any     3.4 KiB         0.0%
```

Following this change, the UI thread decreased from 13% in idle to
0.1% usage, as reported by `tasks`:

```
 → tasks
name            core    free stack      run time
IDLE1           1       1.0 KiB         99.8%
IDLE0           0       1.1 KiB         99.4%
console_repl    any     6.4 KiB         0.5%
ui              any     10.1 KiB        0.1%
btController    0       2.1 KiB         0.0%
Tmr Svc         any     1.4 KiB         0.0%
esp_timer       0       3.4 KiB         0.0%
audio_dec       any     19.4 KiB        0.0%
ipc1            1       1.0 KiB         0.0%
ipc0            0       1.0 KiB         0.0%
hciT            1       1.5 KiB         0.0%
BTU_TASK        1       3.2 KiB         0.0%
worker_3        any     20.7 KiB        0.0%
worker_2        any     29.4 KiB        0.0%
BTC_TASK        1       2.5 KiB         0.0%
worker_1        any     30.1 KiB        0.0%
worker_0        any     5.7 KiB         0.0%
lvglDraw        any     2.9 KiB         0.0%
main            0       2.0 KiB         0.0%
audio_conv      any     3.4 KiB         0.0%
```

src/tangara/ui/lvgl_task.cpp

@@ -63,7 +63,7 @@ auto UiTask::Main() -> void {
     }
 
     TickType_t delay = lv_timer_handler();
-    vTaskDelay(pdMS_TO_TICKS(std::clamp<TickType_t>(delay, 0, 100)));
+    vTaskDelay(pdMS_TO_TICKS(std::clamp<TickType_t>(delay, 16, 100)));
   }
 }