feat: network-aware coin name and unit strings
Add COIN_SYMBOL/COIN_NAME to AbstractNet (defaults: BTC/Bitcoin).
BitcoinPurple overrides to BTCP/Bitcoin Purple; testnet inherits.
Replace module-level base_units/base_units_list in util.py with
get_base_units()/get_base_units_list() that read constants.net.COIN_SYMBOL
at runtime, producing [BTCP, mBTCP, bits, sat] on BitcoinPurple.
Update all UI touch points: Qt window title, watching-only warning,
invalid address message, testnet warning, settings unit combo + help
text, QML networkName property, and Preferences thousands-separator label
davide
d51076cb0c