feat: add German (QWERTZ) keyboard layout support

[NakaokaRei]

Overview

Add support for the German QWERTZ keyboard layout (ISO physical keyboard).

Major differences from US QWERTY:

Char	US	German
y / z	Swapped positions (QWERTZ)
@	Shift+2	Option+L
[	[ (0x21)	Option+5
]	] (0x1E)	Option+6
{	Shift+[	Option+8
}	Shift+]	Option+9
\	\ (0x2A)	Option+Shift+7
ä	N/A	' key position
ö	N/A	; key position
ü	N/A	[ key position
ß	N/A	- key position

Notes

• German layout requires Option (Alt) key for many symbols, which is a new modifier pattern not currently handled
• Y and Z keys are physically swapped compared to QWERTY
• Has dedicated keys for umlauts (ä, ö, ü) and ß

Tasks

• Add .german case to KeyboardLayout
• Handle Option key modifier in CharacterKeyMapping (may need needsOption field)
• Implement German character mapping
• Add tests
• Update README

Related

• PR feat: add JIS keyboard layout support #56 (JIS keyboard layout support)