feat: support sending browser content and saving CSS selectors to webhook

[abdefi]

This pull request introduces a new selector capture feature to the extension, allowing users to visually select elements on a webpage and save their selectors for use with webhooks. It adds a dedicated content script for selector capture, updates localization files with relevant UI strings and error messages in both English and German, and modifies the extension manifest to register the new content script and required permissions.

The most important changes are:

Selector Capture Feature Implementation:

• Added a new content script content-scripts/selector-capture.js that enables users to enter a capture mode, visually highlight elements, generate unique selectors, and persist them for a given webhook. The script handles user interactions, selector uniqueness, error cases, and communicates with the extension via runtime messages.

Manifest and Permissions Updates:

• Updated manifest.json to register the new content script for all URLs, and added the necessary permissions (scripting, tabs) to enable selector capture functionality.

Localization and UI Strings:

• Added comprehensive English and German localization entries in _locales/en/messages.json and _locales/de/messages.json for selector capture UI, error messages, tooltips, and status notifications, supporting a user-friendly and accessible experience.