Add side panel open behavior option

[imadreamerboy]

I added the side-panel as an additional option for the opening behaviour. As part of that i reworked the background controller and funneled every entry point (toolbar, context menu, highlighter) through a single helper function, this ensures that the side panel is opened only in direct response with user gestures.

I successfully tested the functionality on Chrome, Edge, Vivaldi, Firefox.

Open behavior enhancements:

• Added "sidepanel" as a new value for the openBehavior setting throughout the codebase, including types, settings management, and storage utilities. [1] [2] [3] [4] [5]
• Updated the settings UI (settings.html) and localization to allow users to select "side panel" as the default open behavior. [1] [2]

Background script and browser integration:

• Implemented robust logic in background.ts to handle opening the Clipper UI in the side panel using the appropriate browser APIs for Chrome, Firefox, and other browsers, including error handling and fallback behaviors. [1] [2] [3] [4] [5] [6] [7] [8]
• Updated context menu and message handling to support side panel and embedded modes, including new actions and improved tab resolution.

Manifest and browser-specific configuration:

• Added sidebar_action to the Firefox manifest to enable the side panel feature in Firefox.

Popup and UI logic:

• Updated popup logic to respect the new open behavior, opening in a side panel when selected and the environment supports it.
• Improved error handling and messaging for opening the Clipper UI from various entry points, including the highlighter button.

[imadreamerboy]

as we discussed in #498 i implemented the side-panel behaviour on top of your existing solution.

I have not implemented a designated shortcut to open the side-panel. If we want/need that it's something i can quickly add, open for suggestions or improvements.