Add a scrollbar to BookContent

[kdroidFilter]

Problem

BookContent uses virtualized rendering (LazyColumn + Paging 3) which means only visible lines are rendered. Without knowing the actual height of all content, a traditional scrollbar cannot accurately represent the user's position in the document.

A naive index-based scrollbar (position = current line / total lines) fails because line heights vary dramatically.

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Data Analysis

The database contains ~4 million lines with extreme content length variation:

Content Length	Line Count	Estimated Visual Lines
0 (empty)	18	0
1-99 chars	1,815,424	~1
100-499 chars	1,321,836	1-5
500-999 chars	450,538	5-10
1000-4999 chars	353,235	10-50
5000-9999 chars	14,649	50-100
10000+ chars	3,956	100+
Maximum	1	1,275,307 chars ≈ 15,000 visual lines

Example: At database line 50/100, an index-based scrollbar shows 50%. But if lines 1-49 are short (1 visual line each = 49 lines) and lines 50-100 are long (50 visual lines each = 2,550 lines), you're actually at 49 / 2,599 ≈ 2% of the document.

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Proposed Solution

1. Store character count per line in the database with a cumulative index
2. At runtime, measure how many characters fit per visual line for the current view width
3. Calculate scroll position using: position = cumulative_chars_at_line / total_book_chars

This must be reactive to view size changes (resize, zoom, font changes).

Limitations

This approach will not be pixel-perfect since bold or italic characters take slightly more horizontal space than regular text, affecting line wrapping. However, this difference should be negligible and not perceptible to users during normal navigation.

Benefits

• Scrollbar accurately reflects real content position
• Works with fully virtualized content (no need to render everything)
• O(1) position calculation with pre-computed cumulative index