feat: support hardware keyboard input, including numpad keys

[MiMoHo]

Type of change(s)

• Bug fix
• Feature / enhancement

What changed and why

The Calculator had no hardware-keyboard handling at all — MainActivity only wired the on-screen button click listeners, and the formula/result views are non-editable AutofitTextViews (no IME path). So every key event from a physical/numpad keyboard (the case reported in #134, e.g. an 8BitDo/Clicks numpad) was silently dropped, whether top-row KEYCODE_0..9 or KEYCODE_NUMPAD_*.

Fix: add an onKeyDown override that maps both the top-row and numpad key codes — digits, + − × ÷, =, Enter, decimal, backspace, clear — to the exact same calc.* calls the on-screen buttons already use (numpadClicked, handleOperation, handleEquals, handleClear, handleReset). Any unrecognized key falls through to super.onKeyDown, so no existing behavior changes.

Tests performed

Built fossDebug, installed on an Android 15 (API 35) emulator, drove input via adb shell input keyevent:

• Numpad digits 7 8, NUMPAD_ADD, 2, NUMPAD_ENTER → evaluates correctly.
• ESC resets the display to 0; then NUMPAD_5, NUMPAD_MULTIPLY, NUMPAD_3, NUMPAD_ENTER → 5×3 = 15.
• Top-row digits (KEYCODE_7/8/9) enter digits; KEYCODE_DEL deletes the last character (backspace).
• Unmapped keys (Back, volume) behave normally.

Closes the following issue(s)

• Closes Calculator does not register numbers entered on hw keyboard (Clicks case for Razr) #134

Checklist

• I read the contribution guidelines.
• I manually tested my changes on device/emulator.
• I updated the "Unreleased" section in CHANGELOG.md.
• I have self-reviewed my pull request (no typos, formatting errors, etc.).
• I understand every change in this pull request.

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