fix: align currency and calc widget with ios

[piotr-iohk]

Fixes #881

Description

This PR:

1. Shows currency symbols in the "Other Currencies" section of the local currency settings (previously only currency codes were displayed)
2. Truncates 3+ character fiat symbols in the calculator widget icon badge to prevent layout overflow (matching iOS behavior from bitkit-ios#341 / bitkit-ios#513)
3. Pre-fills new calculator widgets with a default BTC value of 10 000 and auto-derives the fiat equivalent on first load
4. Removes auto-clear-on-focus behavior from both BTC and fiat inputs so values persist when switching between fields
5. Switches fiat input keyboard to decimal type and uses space as grouping separator instead of comma

Preview

QA Notes

1. Currency settings symbols

1. Go to Settings -> Local currency
2. Scroll to "Other Currencies"
3. Verify each currency shows its symbol in parentheses (e.g. "AED (د.إ)")
4. Verify "Most Used" section still shows symbols as before

2. Calculator widget symbol truncation

1. Add a calculator widget on the home screen
2. Change local currency to CHF or XDR
3. Verify the fiat icon badge shows a single character ("C" or "X"), not the full code
4. Change to a 1-2 char symbol currency (USD, EUR, PLN) and verify it renders normally

3. Calculator widget default value

1. Remove existing calculator widget (if any)
2. Add a new calculator widget
3. Verify BTC field shows "10 000" and fiat field shows the converted amount
4. Verify both fields are editable without clearing on focus

4. Calculator input behavior

1. Tap fiat field, type a value (e.g. "12.34") — verify decimal input works
2. Tap BTC field — verify fiat value is preserved (not cleared)
3. Tap fiat field again — verify BTC value is preserved
4. Delete all in fiat field — verify you can type a new value including "."
5. Verify fiat uses space grouping (e.g. "1 234.56" not "1,234.56")

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