fix: Area Measurement units and MeasureMark visibility

[MiguelG97]

Description

This PR fixes two related issues affecting area/volume measurements and their label visibility (closes #668):

BEFORE:

AFTER:

1. Incorrect unit conversion scaling:
   Area and volume are scaled incorrectly, causing small measurements (e.g. m2 -> km2) to collapse to 0 even with high precision (5).
   The unit conversion utility already defines factors for area (m², km²) and volume (m³, km³) in the unitFactors variable.
   ```
   const unitFactors: Record<string, number> = {
   // Length
   m: 1,
   cm: 0.01,
   mm: 0.001,
   km: 1000,
   // Area
   m2: 1,
   cm2: 0.0001,
   mm2: 0.000001,
   km2: 1000000,
   // Volume
   m3: 1,
   cm3: 0.000001,
   mm3: 0.000000001,
   km3: 1000000000,
   };

However, the conversion logic applied an additional exponentiation step, effectively scaling the values twice.

```
if (fromUnit.endsWith("2") && toUnit.endsWith("2")) {
      factor **= 2;
    } else if (fromUnit.endsWith("3") && toUnit.endsWith("3")) {
      factor **= 3;
    }
```

2. MeasureMark visibility:

   The MeasureMark visibility was determined using the area’s value property. Since value applies unit conversion and rounding (default precision: 2), small measurements (e.g. m² → km²) were converted to 0. As a result, the label were turned off, negatively affecting the user experience. The solution is to use the rawValue instead, which preserves the original area value in m².

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following:

• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Follow the Conventional Commits v1.0.0 standard for PR naming (e.g. feat(examples): add hello-world example).
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.