Fix habit streak counter to require full daily completion of targetGoal

[Copilot]

Streak was incremented on every individual log entry regardless of targetGoal, and had no consecutive-day validation — logging on day 1 and day 3 would produce a streak of 2.

Changes

• calculateHabitStreak() (new) — derives streak by grouping logs by date, filtering to days where count >= targetGoal, then walking backward through sorted dates counting consecutive days from the most recent fully-completed day
• logHabitCompletion() — replaces streak + 1 with a call to calculateHabitStreak() on the current log state, so retroactive completions (filling a gap in past days) are correctly reflected

// targetGoal: 3 — partial completion no longer advances streak
logHabitCompletion(habit.id, today); // count=1, streak=0
logHabitCompletion(habit.id, today); // count=2, streak=0
logHabitCompletion(habit.id, today); // count=3, streak=1 ✓

// Retroactive gap-fill
logHabitCompletion(habit.id, day1); // streak=1
logHabitCompletion(habit.id, day3); // streak=1 (gap at day2)
logHabitCompletion(habit.id, day2); // streak=3 ✓

Original prompt

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This section details on the original issue you should resolve

<issue_title>Streak counter incorrect</issue_title>
<issue_description>When daily streaks are calculated for habits, it should be the number of days that the entire habit was completed. For example, if a user has Drink Water and it can be repeated 10 times that day, it should only count as a streak if they get all available units completed

If the user goes back to previous days to complete their habits, the streak should continue if they mark all units as completed for that habit</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Streak counter incorrect #24

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