Move custom views to uhabits-core

[iSoron]

Description

To make Loop Habit Tracker available on iOS and the web, we need to be able to render custom views in multiple platforms. Currently, we already have a multiplatform canvas implementation in org.isoron.platform.gui.Canvas (in uhabits-core). While a few custom views are already using this multiplatform implementation (including BarChart and HistoryChart), most of our custom views still use android.graphics directly.

This task is to migrate all remaining custom views to our multiplatform canvas, and remove all direct references to android.graphics.

Notes

1. We already have some partial implementations (not currently used) in org.isoron.uhabits.core.ui.views, including CheckmarkButton, NumberButton and Ring. These implementations need to be finished.

Subtasks

• activities/common/views/FrequencyChart.kt
• activities/common/views/RingView.kt
• activities/common/views/ScoreChart.kt
• activities/common/views/StreakChart.kt
• activities/common/views/TargetChart.kt
• activities/habits/list/views/CheckmarkButtonView.kt
• activities/habits/list/views/HeaderView.kt
• activities/habits/list/views/NumberButtonView.kt