Implement Markdown export for the entire inventory (Issue #36).
The method renders categories as headings with items in a table
(Name, Menge, Einheit, MHD, Lagerort). Empty categories are skipped.
Dates are formatted as dd.MM.yyyy (German), quantities use German
decimal format (comma). Settings section shows household_size and
kcal_per_day if present.
Includes 6 unit tests covering: full export, empty categories,
missing expiry date, settings section, fractional quantities, and
irrelevant settings omission.
Closes#36
12f787a406