feat: add css classes for context colors

[phortx]

Why

I'd love to give the context tags different colors via CSS. Unfortunately this is not possible currently as the elements are not distinguishable.

What

I extended the .context-tag elements with a BEM modifier class (context-tag--color-X) that enables CSS-based coloring based on the context name.

That allows users to style their context tag elements. Also it is the very first step to bring configurable colored context elements as a feature in TaskNotes in the future.

Changes

• Added getContextColorClass() function in src/ui/renderers/tagRenderer.ts that generates consistent color classes using a djb2 hash algorithm
• Context tags now receive a modifier class like context-tag--color-0 through context-tag--color-19 (so we get 20 distinct colors)
• Same context name always produces the same color class (case-insensitive, with/without @ prefix)
• Added unit tests

Usage

CSS can now target specific color variants:

.context-tag--color-0 { background-color: red; }
.context-tag--color-1 { background-color: blue; }
/* ... etc */

[callumalpass]

Thanks for this PR, @phortx. Sorry it has taken a while to review. I'm happy to merge it and it will be included in the next release.