[Feature Request] Support for "Sub-Intervals" or "Timed Exercise Lists" within a Work Block

### Checklist

- [x] I made sure that the issue I am raising doesn't already exist

### Use case / Problem

I am currently using OpenHIIT for long-form interval training and noticed a limitation when a single "Work" interval consists of multiple consecutive tasks.

**The Problem:**
When a "Work" interval consists of several different exercises (e.g., a 180s block split into three 60s tasks), the app lacks the granularity to guide the user through these changes:

* **Lack of Visual Feedback:** Since the "Exercise Label" is static, it cannot highlight the currently active task. Users have to resort to long strings like `"1. Burpees | 2. Plank | 3. Squats"`.
* **Manual Tracking:** The user has to constantly monitor the countdown and perform mental math to know when to switch (e.g., *"I need to switch when the timer hits 2:00, and again at 1:00"*). This breaks the flow and focus of the workout.
* **Missing Audio Cues:** There is no audio signal to indicate the transition between tasks within the same work block.

**Proposed Solution: Sub-Intervals**
I suggest allowing a single "Work" interval to be divided into **Sub-Intervals** (or a Timed Exercise List).

**Example:**
* **Total Work Interval:** 180s
* **Sub-Interval 1 (60s):** "Burpees" -> *Display only this label for the first 60s.*
* **Sub-Interval 2 (60s):** "Plank" -> *Display only this label for the next 60s.*
* **Sub-Interval 3 (60s):** "Squats" -> *Display only this label for the final 60s.*

**Benefits:**
1. **Visual Clarity:** The screen only displays the specific task relevant to the current sub-timer.
2. **Audio Cues:** The app could provide a short audio signal (e.g., a beep) during the transition between sub-intervals.
3. **Better Focus:** Eliminates the need for mental math during high-intensity blocks.

### Proposal / Solution

**Technical Suggestion (Data Model):**
The `activities` array could be expanded to support objects with durations:

```json
"activities": [
{ "name": "Burpees", "duration": 60 },
{ "name": "Plank", "duration": 60 },
{ "name": "Squats", "duration": 60 }
`

### Platform

_No response_

### Audio Files/URLs/Sources

_No response_

### Mockups

_No response_

### Related issues / more information

_No response_

### Working on PR

nope