Add configurable source-key swipe actions

[AsafMah]

Summary

• Add a SourceKeySwipeActions binding model for ACTION/Enter, period, and comma source keys.
• Dispatch configured swipes through existing toolbar/key-code actions instead of adding new action paths.
• Add an Advanced setting for the MVP binding string, e.g. ACTION:LEFT=JOIN_NEXT;PERIOD:UP=UNDO_WORD.
• Add focused parser/canonicalization tests and changelog entry.

Notes

• This deliberately does not add Enter/period/comma to KeyCode.isModifier().
• Shift/Symbol/Numpad sliding remains hard-coded in KeyboardState.
• Space/delete swipe paths remain excluded from the source-key swipe binding path.

Verification

• ./gradlew.bat :app:testOfflineDebugUnitTest --tests "helium314.keyboard.latin.utils.SourceKeySwipeActionsTest" --tests "helium314.keyboard.settings.SettingsContainerTest" --no-daemon --console=plain
• ./gradlew.bat :app:assembleStandardDebug --no-daemon --console=plain

Closes #37

[AsafMah]

Closing this because it implemented the wrong abstraction: coarse (source key + direction) shortcuts. #37 is about Shift/Symbol/Numpad-style press-slide-release selection, where the target under the finger supplies the action. The issue body has been corrected to reflect that.