[BUG]: Add validation for invalid date ranges in Contributor Profile

### Bug Description

The Contributor Profile page currently allows users to select an invalid reporting date range where the Start Date is later than the End Date.

For example:

* Start Date: 20-Aug-2026
* End Date: 10-Aug-2026

Currently, the page accepts this invalid date range without showing any validation message and treats it as an empty reporting period.

This can be misleading because the user may think that the contributor had no activity during the selected period, instead of being informed that the selected date range is invalid.

### Steps to Reproduce

* Go to the Contributor Profile page.
* Open the Reporting Window & Date Presets section.
* Set the Start Date to a date later than the End Date.
* For example, set Start Date to 20-Aug-2026 and End Date to 10-Aug-2026.
* Observe the contribution metrics and contribution table.
* The page displays 0 contributions, 0 pull requests, 0 issues, and 0 active repositories instead of showing a date validation message.

### Logs and Screenshots

No console errors or application logs are produced for this issue.

Current Behaviour

The current UI treats the invalid date range as a valid empty reporting period.

Expected Behaviour
The page should clearly display a validation message such as:
"Invalid date range: Start date cannot be later than end date."
The invalid range should not be treated as a valid reporting period.

### Environment Details

OS: Windows 11
Browser: Google Chrome
Application: OrgExplorer
Page: Contributor Profile
Reporting Date Range: Start Date later than End Date
No console errors observed

### Impact

Low - Minor inconvenience

### Code of Conduct

- [x] I have joined the [Discord server](https://discord.gg/hjUhu33uAn) and will post updates there
- [x] I have searched existing issues to avoid duplicates

Start at the Contributor Profile page, specifically the Reporting Window & Date Presets section, and reproduce the case where the Start Date is later than the End Date. Trace the date-range handling from that UI; done means an invalid-range validation message is shown and the range is not treated as an empty reporting period.

Vom Indexierungsmodell aus dem Issue-Text verfasst.