feat(support): add support issue templates (DEV-4571)

[bernd2022]

Summary

• New /templates screen and dashboard button to manage bilingual reply templates (DE required, EN optional)
• Template picker inside a support issue with placeholder syntax (\$userData.firstname, \$transaction:last.id, \$issue.uid, …), live preview, language auto-pick from the customer locale, and an array-selection dialog when multiple transactions match
• Send button now blocks while the message still contains unresolved placeholders
• Modal gained a maxWidthClass prop so the picker can render wider than the default dialog

Companion API PR: DFXswiss/api#3728

Test plan

• Create / edit / delete templates from /templates (incl. bilingual content)
• Picker shows preview in customer language, falls back to DE if EN missing
• \$transaction.id opens the array picker when the customer has multiple transactions; \$transaction:last.id resolves directly
• Send button stays disabled while unresolved placeholders remain in the textarea
• Lazy-load of user data triggers only on first template-picker open per issue