Fix bugs and increase test coverage to 91%

[tirthpatell]

Summary

• Fix: Editor cursor scroll on line wrap — Left/Right arrow keys now call scrollToCursor() when wrapping across lines, preventing the cursor from going off-screen
• Fix: Unicode search backspace — Viewer search input backspace now uses rune-based truncation instead of byte-based, fixing corruption of multi-byte Unicode characters
• Remove dead code — Removed unreachable len(lines) == 0 check in NewBuffer (strings.Split always returns ≥1 element)
• Increase test coverage from 74.2% to 91.3% — Added 41 new tests covering edge cases, boundary conditions, and previously untested code paths

Coverage

Package	Before	After
internal/editor	67.0%	95.4%
internal/viewer	76.6%	95.4%
internal/markdown	83.3%	91.7%
internal/linter	97.0%	98.0%
internal/stats	96.8%	96.8%
Total	74.2%	91.3%

Test plan

• All 90 tests pass with -race flag
• go fmt ./... clean
• go vet ./... clean
• Coverage verified via go tool cover -func