Multi task feature

[coryzoon]

Summary

Adds support for multi-task workflows on mobile (volunteer answers 2+ tasks in sequence before a single
classification POST) and refactors the four classifier screens into a single unified shell to make that
possible.

What changed

Phase 1 — Classifier refactor (prep work for multi-task)

The four legacy classifier screens (Question, MultiAnswer, Drawing, Swiper) previously each owned their own data
loading, layout, and submission logic. They've been replaced with a single ClassifierScreen shell that
dispatches to one of four prop-driven workflow body components (SingleChoice, MultiSelect, Drawing,
Swipe).

• New hooks for shared concerns: useSubjectQueue, useWorkflowResources, useProjectTranslations,
  useFeedbackFlow.
• New components replace connect()ed legacy versions where the new prop-driven flow needs them (drawing
  canvas, tool panel, subject viewer, modal sheet).
• New plain-async submission helpers replace the legacy classifier-submission thunks for the new flow. Legacy
  thunks stay in place so the legacy screens still work as a rollback option.
• Legacy classifier screens are kept in the navigator but unreached — intentional, for A/B rollback if anything
  regresses.

Phase 2 — Multi-task chain

• taskChain.js — pure helpers (isMultiTaskWorkflow, getNextTaskKey, isLastTaskInChain,
  activePathTaskKeys) that decide whether a workflow chains and where each task points next.
• classification RTK slice extended with currentTaskKey, taskHistory, and annotationsByTask so each
  chain task's annotation is stored, restorable on Back, and prunable on re-branch.
• ClassifierScreen reads currentTaskKey from the slice and remounts the correct body on chain advance.
  Subject queue lifted into the shell so a Swipe → non-Swipe advance keeps the active subject.
• ChainBackButton renders whenever taskHistory.length > 0. Re-picking a different branch after Back
  automatically prunes stale downstream annotations.
• Submit button labels flip between "Submit" / "Next" / "Done" based on the chain's position; MultiSelect "Done"
  is gated on a non-empty selection mid-chain (matches FEM).
• Single-task workflows are unchanged — POST shape is byte-equivalent to before this PR.
• New i18n keys (next, done, back) in en/mobileEn.json; other locales fall back to inline English.

Notes for review

• HAVI test project bypass: src/actions/projects.js has three // TEMP: patches that force the HAVI test
  project (32778) and workflow (31723) through the project/workflow pipeline despite mobile_friendly: false.
  These must be removed once the backend marks HAVI as mobile-friendly and isValidMobileWorkflow is relaxed to
  accept multi-task chains.
• Legacy classifier screens are intentionally kept for rollback / A/B. Will submit follow up PR for removal.