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:
   • [BUG] Calc widget fiat currency truncated bitkit-ios#341
   • fix(widgets): fix calculator widget keyboard dismiss 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. Hardens calculator field input by sanitizing pasted and typed values for BTC/fiat and preventing classic-mode fiat from being rehydrated back to zero after clearing
6. Switches fiat input keyboard to decimal type and uses space as grouping separator instead of comma

Preview

Screen.Recording.2026-04-03.at.17.45.25.mov

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

Screen_recording_20260423_071500.webm

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

Screen_recording_20260423_073524.webm

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")

Screen_recording_20260423_080014.webm

5. Calculator paste and clear edge cases

1. In MODERN denomination, paste malformed BTC input containing punctuation (for example 1000087188..........,,,,,)
2. Verify BTC shows only the sanitized sat value and does not display pasted punctuation
3. In CLASSIC denomination, clear fiat completely and verify it stays empty instead of snapping back to 0
4. Paste 8 into the empty fiat field in CLASSIC denomination and verify it behaves consistently

Screen_recording_20260423_081652.webm