fix: open correct contact when reopening details after a call

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Summary

Fixes #783 — picking a contact from Favorites/Contacts after calling a previously-viewed contact opens that previous contact instead of the one just tapped.

Root cause

startContactDetailsIntent in ActivityExt.kt launches ACTION_QUICK_CONTACT via the commons launchActivityIntent, which adds only FLAG_ACTIVITY_NEW_TASK. With that flag alone, if the receiving Contacts app's task is still alive from a previous view, Android brings the existing task forward with its old activity stack. The new intent is dropped (the receiver isn't singleTop / singleTask), so the previously-viewed contact is shown.

This is easy to miss in casual testing — pressing back from the quick-contact view finishes the activity and tears down the task, so the bug only reproduces when the previous quick-contact view is left in the background (the typical flow being: open contact → tap call → end call → return to phone app → tap a different contact).

Fix

Add FLAG_ACTIVITY_CLEAR_TASK so the receiver's task is cleared before launching the new view. Applied to both branches (Fossify Contacts via ACTION_VIEW, system Contacts via ACTION_QUICK_CONTACT). The launchViewContactIntent call was inlined so the flag could be set before launchActivityIntent adds NEW_TASK.

Test plan

• Open Favorites, tap contact A → call A → end call → tap contact B in Favorites → contact B's details appear (previously: contact A reappeared)
• Repeat the call/return cycle several times → no stale stack of past contacts builds up
• Plain "tap A → back → tap B" path still works
• Same flow from the Contacts tab
• Same flow with Fossify Contacts installed (private-contact branch)