Add Core Logic for Section Length Validation

Breaking down #13 into parts - this is the first sub-task for setting up the core of the validation feature.

This is to create the foundational, non-API logic for validating resume section lengths. This involves adding the configuration and a helper function that will perform the checks.

For context, this logic will be used by an API endpoint (to be created in a future task) that supports a two-step validation flow:

- Real-time checks (e.g., when a user clicks away from a field), where the frontend will send the full YAML document for validation.
- A final check before a user generates a resume, which also validates the full document as a final safety measure.

**Acceptance Criteria:**

- [ ] **Add a Flexible Configuration Dictionary**
- In [`app.py`](https://github.com/aafre/resume-builder/blob/main/app.py), create a new `RECOMMENDED_LENGTHS` dictionary. This configuration will drive all validation using a prioritised rule system:
1. **By Name (Overrides):** The `by_name` key sets a specific length for a section with an exact `name` (e.g., "Summary"). This rule must take the highest priority.
2. **By Type (Defaults):** The `by_type` key sets a default length for all sections of a certain `type` (e.g., all `text` sections). This is the fallback if no `by_name` override is found. You can find all supported types documented in the [advanced section](https://github.com/aafre/resume-builder/tree/main?tab=readme-ov-file#advanced-usage) of the README.
- Here is an example structure to follow:
```python
RECOMMENDED_LENGTHS = {
# Priority 1: Check for a specific section name for overrides.
'by_name': {
'Summary': 1000,
'Professional Profile': 1000, # Handles common variations
},
# Priority 2: If no name matches, fall back to the section's type.
'by_type': {
# Default for any simple text block
'text': 1500,

# Applied to each bullet point in a list
'bulleted-list': 250,

# Applied to each skill in a horizontal list
'inline-list': 50,

# Special key to target the 'description' field within each 'experience' item
'experience.description': 1200,
}
}
```

- [ ] **Create a Validation Helper Function**
- Implement a new, pure Python function, e.g., `validate_section_lengths(data, config)`.
- It should take parsed YAML data and the config dictionary as arguments.
- It must dynamically iterate through sections and handle the different content structures of types like `text`, `bulleted-list`, and `experience`, applying the length limits from the config.
- It should return a `list` of warning objects for any validation failures.

> **Note:** This task does **not** involve creating a Flask route or handling web requests. We are only building the reusable core logic in this step.