feat: calculator v61 redesign & os widget

[ovitrif]

Fixes #937

Refreshes the Bitcoin Calculator widget for the v6.1 widget design and brings it into the Android OS widget system, refactored to route its ui logic to the viewmodel and improve formatting with locale thousands and decimal separator support for fiat input.

Description

• Updates the in-app calculator widget to the v6.1 treatment, matching the current widget surface, spacing, focus, and input patterns used by the other refreshed widgets.
• Adds Android home screen calculator widgets for the compact and wide variants, including launcher picker previews and tap-to-open behavior.
• Replaces the old calculator text entry behavior with the Bitkit number pad flow: localized decimal entry, stable grouping, delete behavior, cursor styling, fractional zero placeholder, and red invalid-key feedback.
• Polishes calculator focus and dismissal behavior on the app home screen so the number pad, bottom tab bar, scrolling, and navigation bar insets behave together.
• Adds the user-facing changelog fragment for the calculator widget refresh.
• Displays fiat amount with the same formatting from the activity list items.

Design refs:

• Calculator widget wide
• Calculator widget small

Preview

OS Widget	In-App Widget
OS widget picker, OS home screen widget, and tap-to-open behavior.	In-app widget, Numberpad, focus, dismissal, formatting, and tab-bar autohide.
calc_os.mp4	calc_app.mp4

QA Notes

Manual Tests

Dev verified locally:

• Added the calculator widget to the app home screen and verified the v6.1 card renders correctly in idle and focused states.
• Focused both Bitcoin and fiat inputs and verified Bitkit's number pad opens instead of the Android soft keyboard.
• Typed multi-digit Bitcoin and fiat values and verified the cursor stays at the end of the visible value.
• Verified Bitcoin grouping uses the same space-separated style as Bitkit amount views.
• Verified fiat grouping uses normal amount formatting without the unwanted space after separators.
• Verified deleting a digit from a grouped fiat value keeps the remaining value and converted Bitcoin value stable.
• Verified decimal entry uses the localized decimal key and does not show both comma and period options.
• Verified fiat one-decimal input shows the dimmed trailing zero placeholder without layout jump.
• Verified invalid number-pad input gives red button-text feedback through the number pad error state.
• Verified tapping outside the calculator collapses the number pad smoothly.
• Verified the bottom tab bar hides and returns in sync with the number pad animation.
• Verified the center scan button does not flash as highlighted while the tab bar moves.
• Verified the calculator remains visible when the lower fiat input is focused near the bottom of the widget list.
• Verified the number pad leaves room for the Android gesture navigation handle.
• Verified scrolling gestures around the focused calculator do not unexpectedly dismiss the number pad.
• Verified tapping the app menu or moving away from the focused calculator dismisses the number pad.
• Verified the Android widget picker preview uses the expected widget background and row sizing.
• Verified the Android home screen calculator widget has balanced vertical spacing in compact and wide sizes.
• Verified tapping the Android home screen widget opens Bitkit without auto-focusing the calculator input.

Reviewer / QA requested:

• 1. Add the calculator widget inside Bitkit and check idle, focused Bitcoin input, and focused fiat input states.
• 2. Enter and delete large Bitcoin and fiat amounts, especially values above 1,000, to confirm grouping and cursor behavior.
• 3. Check decimal behavior on BE locale for comma decimals and US locale for period decimals.
• 4. Focus the bottom input when the calculator is low in the app home widget list and confirm the number pad remains usable above the OS bottom bar.
• 5. Tap outside the calculator and confirm the numpad collapse and bottom buttons return animate together.
• 6. Repeat focus and dismiss several times and confirm the scan button does not appear highlighted during tab bar movement.
• 7. Add compact and wide calculator widgets from the OS widget picker and compare picker previews with the actual home screen widgets.
• 8. Tap the Android home screen calculator widget and confirm it opens app home screen.

Automated Tests

Added in this PR:

• CalculatorViewModelTest: covers stored value hydration, legacy value backfill, BTC input sanitization/conversion/persistence, fiat input sanitization/conversion/persistence, fiat conversion storage without grouping, currency refresh, and display-unit changes.
• CalculatorCardStateTest: covers calculator formatting helpers, money separators, fiat placeholder zero behavior, hydration guards, and number-pad input/delete behavior.

Integration (Android tests) added via stacked PRs:

• test: add android test lanes + disable claude reviews #943
• test: add calculator widget device tests #944

Automated Checks

• Local: ./gradlew compileDevDebugKotlin
• Local: ./gradlew testDevDebugUnitTest
• Local: ./gradlew detekt

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