fix: correct chunk overlap calculation to use configured overlap

[echobt]

Description

This PR fixes an issue where chunk overlap was calculated using an arbitrary magic number (40) division, which resulted in minimal and inconsistent overlap regardless of the configuration.

Changes

• Replaced the chunk_overlap / 40 logic with a character-based overlap calculation.
• The new logic walks backwards from the current line to find a starting point that satisfies the chunk_overlap character count (or slightly exceeds it to include complete lines).
• Updated both src/core/indexer.rs and src/watcher.rs.

Verification

• Verified that the overlap now properly respects the configured character count (default 64) rather than being 1-2 lines.
• Ran existing tests to ensure no regressions.