Fix Bluetooth receipt text regression and add ASCII charset

[osswald]

Summary

• Restore per-line ESC t code-page selection before each text block in ESC/POS receipts, fixing the v1.5.0 regression where some Bluetooth printers printed only the logo (or nothing when logo was disabled).
• Add an ascii charset preset with German transliteration (ä→ae, ö→oe, ü→ue, ß→ss) for Bluetooth printers that ignore code-page commands.
• Expose the new option in Android Bluetooth Drucker → Zeichensatz as ASCII (ae/oe/ue).

Test plan

• Deploy updated Pi backend and reload Pi frontend on Android
• Printer 2: Run Testbeleg drucken with logo on — expect full receipt (logo + text)
• Printer 2: Disable logo in cloud payment receipt profile — expect text-only receipt to print
• Printer 1: Test umlauts with default PC858; if still wrong, switch to ASCII (ae/oe/ue) and confirm readable output
• pi/backend pytest (250 passed)
• pi/frontend vitest (219 passed)
• ./scripts/lint.sh

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