Update README and Add Changeset for WatchdogGradient Theme

[Copilot]

This PR addresses the missing documentation updates for PR #25 which added the new WatchdogGradient theme.

Changes Made

README.md Updates:

• Updated theme count from 10 to 13 (reflecting the actual number of available themes)
• Added WatchdogGradient to the list of available preset themes
• Added example usage showing how to use the new theme with a sample URL

Changeset Addition:

• Created a proper changeset using the @changesets/cli to document the new theme as a minor version bump
• The changeset will ensure the new theme is properly documented in the changelog when released

Example Usage

The new theme can now be used like any other preset theme:

https://gitroll.io/api/badges/profiles/v1/uZxjMB3mkXpQQPskvTMcp0UeqPJA3?theme=WatchdogGradient

The WatchdogGradient theme features a striking gradient background with custom color schemes for badges and UI elements, providing users with another stylish option for their profile cards.

All builds and linting continue to pass successfully.

Fixes #26.

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