feat(curation): persist Dataset Curator panel state across sessions

[lstein]

Stacked on top of #284 — base is lstein/feature/server-side-preferences. GitHub will rebase the base to master automatically when #284 merges.

Summary

• Persist the five Dataset Curator inputs — target count, iterations, method (Diversity FPS / Blocks K-Means), exclude-matches percent, and export path — to the same per-device preferences store introduced in feat(preferences): per-device server-side UI preferences #284.
• Reopening the panel (or revisiting on another day, or from another browser on the same device after a localStorage eviction) restores the values the user left.
• The export path migrates automatically: existing localStorage["curationExportPath"] values are picked up by restoreFromLocalStorage and pushed to the server via the existing reconciliation path on first boot.

What's new

Backend (preferences.py)

• curation_target_count (int, 10..1000)
• curation_iterations (int, 1..30)
• curation_method (Literal["fps", "kmeans"])
• curation_exclude_threshold (int, 1..100)
• curation_export_path was already in the model from feat(preferences): per-device server-side UI preferences #284.

Ranges mirror the HTML input min/max attrs in curation.html so server validation defends against a stale tab posting an out-of-range value after a future HTML change tightens the bounds.

Frontend

• state.js: five new entries in PERSISTED_SETTINGS, five new setters exported, five new state defaults that mirror the HTML defaults.
• curation.js: replaced manual localStorage.getItem/setItem("curationExportPath") calls with state setters. Added a restore-from-state block at the top of setupEventListeners and wired oninput / onchange handlers on the slider, number, iterations, method radios, threshold, and export path. Each handler clamps before calling the setter so out-of-range typed values can't trip server validation.

Test plan

• pytest tests/backend/test_preferences.py — 12 tests, including two new ones (test_curation_fields_round_trip covering all five fields end-to-end; test_curation_invalid_values_return_422 exercising every range bound and the Literal field).
• npm test — full 334/334 suite still green.
• Lint: ruff + eslint + prettier all clean.
• Manual: open Curator panel, change target count / iterations / method / threshold / export path, close and reopen → values persist. Reload page → still persist. Toggle method between FPS and K-Means → persists.
• Manual: existing user with localStorage["curationExportPath"] set from before feat(preferences): per-device server-side UI preferences #284 → reload after both PRs land → path appears in the input.

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