feat: retire is_focus, merge dashboard weekly widget onto due_date

[dholbach]

Summary

(Replaces #285, which GitHub auto-closed when its base branch feat/focus-queue-due-today was deleted after #284 merged — no content change, just retargeted at main, which already contains #284.)

• The dashboard's weekly widget star toggle (is_focus, P-028, week-scoped) and the Focus Queue's "due today" concept (due_date, P-050, added in feat: add a "Today" quick action to pull a Focus Queue task to the top #284) were two separate "important task" signals — exactly the overlap issue Focus Queue vs. dashboard's is_focus flag: overlapping "focus" concepts #281 flagged.
• Turned out the star had already lost its only "add to focus" UI path back in P-050 phase 4 (the /todos/ list page it lived on was retired; only "remove focus" survived, inline in the dashboard widget) — so there was no way to set is_focus=True at all anymore, just an unusable leftover.
• Rather than restoring a second star toggle, is_focus is retired outright (migration 0018_remove_practicetodo_is_focus) and the dashboard's weekly widget now lists due_date <= today tasks — the same signal the Focus Queue itself sorts on via the "Today" quick action from feat: add a "Today" quick action to pull a Focus Queue task to the top #284.
• Removes todo_toggle_focus (view, URL, CSS .btn-focus-toggle/.is-focused) — fully dead once the field is gone.
• Adds test_weekly_focus_widget.py — no test coverage existed for WeeklyFocusWidgetBuilder before this.
• Docs: updated P-028/P-050 project docs with the retirement note, fixed a stale widget-builder inventory line in CODE_STRUCTURE.md (pre-existing staleness from the P-050 phase-4 retirement, adjacent to what this touched), added a FEATURES.md entry, updated PROJECTS.md.

Resolves #281.

Test plan

• ./dev.py test my_practice.tests.test_weekly_focus_widget my_practice.tests.test_views_todo my_practice.tests.test_views_focus_queue my_practice.tests.test_dashboard_widgets my_practice.tests.test_views_dashboard my_practice.tests.test_i18n_coverage my_practice.tests.test_sync_focus_queue_tasks — all pass, including new due-today/overdue/future-due-date/no-due-date/completed coverage for the widget builder
• ./dev.py test (full suite, Django + JS) and ./dev.py review (monthly automated checks: vulture dead-code, ruff unused symbols, pip-audit/npm-audit CVEs) — both clean
• ./dev.py lint — ruff format/lint + Tailwind build all pass
• ./dev.py i18n — fixed two entries makemessages fuzzy-guessed from similarly-worded existing strings
• Manually verified via Django test client against the running dev container: clicking "Today" in the Focus Queue makes the task appear on the dashboard's "Due today" widget with no leftover star button or dead toggle-focus URL in the rendered HTML

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