Add t() translation function to i18n.rs with ~200 key/value pairs
covering navigation, card headers, field labels, buttons, filters,
planting calendar rows, and general UI text in both German and English.
Updated all 11 source files to use t(&lang, "key") instead of
hardcoded English strings: app.rs (sidebar nav), all 7 page files
(species, cultivars, families, suppliers, home, search, sources),
and all 3 component files (planting_calendar, table_controls,
plant_card). Boolean values (Yes/No/Annual) are also translated.
florian.berthold
aa36c846d7