Improve Highlighter Selection and Rendering

There are several issues with the highlighter that seem to be related to how the highlight area is calculated and rendered.

First, the rounded edges at the beginning and end of highlighted words are too large. This creates unnecessary extra space around the text, even when the highlighted word is not next to another highlighted word. Please reduce the size of these rounded edges so the highlight follows the text more precisely.

When multiple adjacent words are highlighted, the rounded edges between them can overlap and create uneven, overly thick, or distorted areas. The inner rounded edges between adjacent highlighted segments should be removed or flattened so that they connect cleanly as one continuous highlight, while keeping a subtle rounded edge only at the outer ends. But when the selection is for a sentence, it is a connected selection.

There is also an issue with automatic highlighting when the user long-presses a word and then presses the highlight button. Depending on the font, character spacing, or letter shapes, the app sometimes treats individual characters as separate elements. This can cause each letter to be highlighted separately, or cause larger and uppercase letters to have a different highlight size from smaller letters, resulting in a distorted and uneven word.

When the user selects a single word and presses the highlight button, the app should recognize it as one text unit and highlight the entire word as a single continuous and consistent area. The highlight should have a consistent height and appearance across the word regardless of differences in letter size, font, or character shape.

Ideally, the highlighter should use the actual text selection boundaries rather than the individual character shapes when calculating the highlight area. This would make the highlighting more accurate, consistent, and visually clean for both individual words and adjacent highlighted words.