fix: align "No content" table with the base and panel count

[MeGaurav4]

An empty file (e.g. /dev/null) renders a "No content" row whose cell widths were hardcoded for the hexadecimal base with two panels, so the row no longer matched the table borders.

Root cause: the empty-file branch forced base_digits = 2 (hex width) and only ever rendered one panel's worth of cells, regardless of the configured base and panel count.

Fix: drop the hardcoded base override and render one cell per panel using panel_sz(), so the "No content" row matches the borders for every base (binary, octal, decimal, hexadecimal) and any panel count.

Only src/lib.rs is changed, plus 3 regression tests for non-hex empty-file output.

Closes #288

Verification

• hexyl /dev/null --base=binary|octal|decimal|hexadecimal all render aligned tables
• Hexadecimal output is byte-identical to before (existing empty_file_passes test passes unchanged)
• cargo test --all: 59 passed (13 lib + 5 parser + 41 integration)