Fix dark-on-dark supervision textarea, merge duplicate Supervision UI

[dholbach]

Summary

• .cn-textarea-small was missing color: var(--color-text-primary) (unlike its -large/-medium siblings), so the supervision-topic textarea rendered dark text on the dark input background.
• The client detail page had two separate "Supervision" add-forms doing overlapping things: a ClientNote note_type="supervision" dated markdown note, and a SupervisionItem open/discussed topic queue. Merged into one workflow: SupervisionItem gains optional resolution_notes/resolved_date, filled in via a new supervision_item_resolve view when a topic is marked discussed. Existing supervision-type ClientNotes are migrated into already-discussed SupervisionItems (migration 0034).
• The Supervision section is now collapsed by default behind a "+ Supervision" toggle button next to "+ Note"/"+ Session log" (matching the existing note-toggle pattern), instead of always being expanded.

Test plan

• ./dev.py test my_practice.tests.test_clinical my_practice.tests.test_admin my_practice.tests.test_i18n_coverage my_practice.tests.test_css_tokens
• ./dev.py test --fast (full Django + JS suite, after clearing stale parallel-worker test DBs)
• ./dev.py lint (ruff format + lint + Tailwind build)
• ./dev.py i18n — no fuzzy entries, all new/changed msgids given real German translations
• Verified via Django test client render that: no stray note_type input remains, the resolve form/URL render for open items, the cockpit "+ Supervision" shortcut correctly toggles the (now-collapsible) section
• Verified the data migration round-trips Fernet-encrypted content correctly (10 existing supervision notes → SupervisionItem rows)

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