feat: add calculator v61 app & 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.

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.

Design refs:

• Calculator widget wide
• Calculator widget small

Related test PR:

• test: calculator widget + dev integration tests setup #934

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 a device locale that uses comma decimals and one that uses 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 system navigation handle.
• 5. Tap outside the calculator and confirm the number pad collapse and bottom tab bar 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 Android widget picker and compare picker previews with the actual home screen widgets.
• 8. Tap the Android home screen calculator widget and confirm it opens Bitkit without automatically focusing an input.

Automated Checks

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

Notes:

• More automated tests to be added in stacked PR.