Support per-instance pinning across multiple Pinned Item widgets

[mapgie]

Summary

This is the last item from the original widget UX investigation: PinnedItemStore was a single global pin, so placing 2+ "Pinned Item" widgets on one home screen always showed the identical item — no way to give each one something different (e.g. one per household member).

• PinnedItemStore gains per-appWidgetId overrides (pinnedItemFor, setPinnedFor, togglePinnedFor) that fall back to the existing default pin when unset. The existing pinnedItem/setPinned/togglePinned API is untouched, so behavior with 0-1 widgets placed is unchanged.
• PinnedItemWidget resolves its own GlanceId to an appWidgetId (GlanceAppWidgetManager(context).getAppWidgetId(id)) and reads that specific instance's pin instead of the shared default.
• The in-app pin toggle (ChoreListViewModel/TaskListViewModel) checks how many Pinned Item widgets are currently placed (AppWidgetManager.getAppWidgetIds): 0-1 behaves exactly as before; 2+ shows a new PinWidgetChooserDialog ("Pin to which widget?") to pick which instance to target, since widgets have no user-given label yet (there's no config screen to set one).
• When exactly one widget is placed, any stale per-instance override left over from a previous 2+-widget state is cleared so that widget reliably tracks the default again.
• Adds a changelog/unreleased/ fragment (minor bump).

Known limitation (documented in code, flagged for follow-up rather than silently shipped): the in-app pin icon/highlight on task and chore cards still reflects only the default pin, not "pinned to any widget instance" — once a household is using 2+ widgets via the chooser, the star icon may not track a pin made to a specific non-default instance. A proper fix wants a real widget config screen with user-given labels, which is a larger follow-up (this PR intentionally stays scoped to the data model + minimal numbered chooser rather than building that now).

Test plan

• With 0 or 1 Pinned Item widgets placed, confirm pinning behaves exactly as before (no chooser, no visible change).
• Place 2 Pinned Item widgets, pin a task via the chooser to "Pinned Widget 1" and a different chore to "Pinned Widget 2"; confirm each widget shows its own item.
• Remove one of the two widgets so only one remains; pin something new and confirm the remaining widget updates (doesn't get stuck on a stale override).
• CI build/lint/changelog checks pass.

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