Fix done status not persisting after reload

[sandervonk]

Summary

Marking a task done in the Vue Portal updated the UI immediately but did not survive a full reload. Root cause was on the Vue client (with API contract notes below).

Cause

1. Silent local stub on failed writes — patchMeTask treated 404 / 405 / 501 as success and returned an in-memory stub. set_finished then applied that stub to task_states and showed a success toast. Nothing was written server-side, so reload cleared done.
2. Fragile path encoding — completion called PATCH /api/v1/me/tasks/:taskId with URL-encoded slash paths (classId%2FtaskId). Live API / Brief docs prefer board tilde refs (classId~taskId). Encoded / is brittle on single-segment Express routes behind Firebase Hosting; Brief batch done works because it sends tilde ids in the JSON body, not as path segments with %2F.
3. Read-path lag — after a real write, the user-doc finished[] snapshot updated account_doc but did not merge into task_states (which is_task_completed prefers).

Fix (Vue only)

• Encode /me/tasks/:taskId with tilde refs (classId~taskId); never put %2F in the path segment.
• Prefer POST .../done / .../undo (and batch POST /api/v1/me/tasks/done|undo for multi-select) for completion — same contract Brief uses.
• Never stub completed mutations; surface errors via the existing ErrorToast.
• Parse done/undo envelopes that return id + numeric completed_at.
• Sync users.finished[] into task_states on snapshot (including undo clears).
• Same tilde encoding for workspace task-path URLs (path param only; no Drive/workspace chrome changes).

Tests

• npm run test:me-tasks — board state merge
• npm run test:paths — tilde encoding / normalize helpers (asserts no %2F)

Follow-up on mvtt-server (no PR there from this change)

• Confirm production has deployed hybrid PATCH /api/v1/me/tasks/:taskId + task_state (functions/api/routes/workspace.ts / services/taskState.ts). README still documents done/undo more prominently than PATCH.
• Optional hardening: reject or redirect slash/%2F path params more clearly, or document that clients must use tilde refs for :taskId (Brief already does).
• Optional: ensure GET /api/v1/me/board tasks[].completed / finished[] stay aligned with hybrid hot maps after undo (Vue now dual-reads both).

No Drive OAuth, nested delete, or unrelated workspace UI changes.