Add zen mode to Tasks, matching Chores

[mapgie]

Summary

Chores already had a zen mode (declutter toggle: no status colours, no due countdown, no category grouping, sorted by most/least overdue). Tasks had no equivalent. This PR adds it.

• New independent taskZenMode DataStore setting (SettingsRepository), separate from Chores' existing zenMode so the two screens can be toggled independently, consistent with how groupChoresByCategory/groupTasksByCategory are already split per screen.
• TaskListScreen: new spa-icon toggle in the filter row (same icon/placement as Chores). While zen is active it replaces the sort/owner-filter icons with a single ascending/descending toggle (most urgent vs least urgent first) and disables category grouping, mirroring the Chores screen's zen behaviour.
• TaskListViewModel: TaskUiState gains zenMode / zenSortAscending. When zen is on, displayed sorts by urgency (overdue -> today -> this week -> later) ascending or descending instead of the regular Priority/Due/Created sort.
• TaskCard: new zenMode param that hides the priority colour bar, hides the category badge, and replaces the colour-coded urgency DueBadge with a plain, uncoloured due date (ZenDueLabel), matching how ChoreCard drops its status colour bar and shows a plain absolute date in zen mode.

Test plan

• Toggle zen mode on the Tasks tab: colour bar and category badges disappear, due dates render as plain "MMM d" text, category grouping collapses into a flat list.
• Toggle the ascending/descending icon in zen mode and confirm overdue/soonest-due tasks move to the top or bottom accordingly.
• Confirm zen mode on Tasks is independent of zen mode on Chores (toggling one does not affect the other).
• Confirm the zen setting persists across app restarts.
• python3 a11y_check.py passes (already verified locally, all clean).

CI (Gradle build/lint/tests) runs the compile check; this container has no Android SDK.

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