feat(highlights): position-based text highlighting

[kdroidFilter]

Summary

• Replace text-matching highlight system with position-based approach
• Highlights now stored with lineId + character offsets instead of matching text
• Only the specific selected instance is highlighted, not all occurrences of the same text
• Add LineTextRegistry to track visible line content for position lookup
• Use deterministic iteration (sorted by lineId) for consistent behavior

Technical Details

• HighlightStore.kt: Refactored to store UserHighlight(lineId, startOffset, endOffset, color)
• LineTextRegistry.kt: New registry tracking plain text of visible lines per tab
• Highlighting.kt: Updated applyUserHighlights to use position offsets
• BookContentView.kt: Filter highlights per line, register line text with registry
• BookContentScreen.kt: Use registry to find position when adding highlights

Note

Highlights are currently in-memory only and will be lost when the app closes.

Follow-up: Persistence Implementation

To persist highlights to UserSettingsDb, the following changes are needed:

1. Create UserHighlights.sq

Location: SeforimApp/src/commonMain/sqldelight/io/github/kdroidfilter/seforimapp/db/

CREATE TABLE IF NOT EXISTS user_highlights (
    id INTEGER PRIMARY KEY AUTOINCREMENT,
    bookId INTEGER NOT NULL,
    lineId INTEGER NOT NULL,
    startOffset INTEGER NOT NULL,
    endOffset INTEGER NOT NULL,
    colorArgb INTEGER NOT NULL
);

CREATE INDEX IF NOT EXISTS idx_highlights_book ON user_highlights(bookId);

selectAllForBook:
SELECT * FROM user_highlights WHERE bookId = ?;

insert:
INSERT INTO user_highlights(bookId, lineId, startOffset, endOffset, colorArgb)
VALUES (?, ?, ?, ?, ?);

deleteOverlapping:
DELETE FROM user_highlights 
WHERE bookId = ? AND lineId = ? 
AND startOffset < ? AND endOffset > ?;

deleteForBook:
DELETE FROM user_highlights WHERE bookId = ?;

2. Create HighlightRepository.kt

Follow the pattern of CategoryDisplaySettingsStore:

• Inject UserSettingsDb
• Async methods for CRUD operations
• Convert Color ↔ Int (using color.toArgb() and Color(argb))

3. Modify HighlightStore.kt

• Inject HighlightRepository
• Load highlights from DB on first access per book
• Persist changes in addHighlight() / removeHighlight()
• Keep in-memory cache for reactive UI updates

Test plan

• Select text and apply highlight color from context menu
• Verify only the selected instance is highlighted, not all occurrences
• Test with text that appears multiple times in the document
• Verify highlight removal works correctly