Summary

The /tasks and /tasks/groups pages previously listed the last 5 recently-viewed items as bare IDs with no context. This PR stores richer metadata at view time and displays up to 20 entries ordered by recency.

Before

• 5 bare task/group IDs with no context about what they were

After

• Up to 20 entries, ordered by when you last visited
• Tasks: shows task name (falls back to ID), status label, task queue, age, and truncated source URL
• Task groups: shows decision-task name, task queue, age, source URL, task count breakdown (completed/failed/running/pending), and a resolved/running indicator

Design decisions

• Store-at-view-time snapshot: metadata is captured when the page is first loaded, not refreshed in the background. Keeps browser storage small and avoids replicating live task tracking (Treeherder territory).
• 20-item limit: up from 5; a viewedAt secondary index on both Dexie tables makes orderBy('viewedAt').reverse().limit(20) efficient.
• Legacy record graceful degradation: existing v1 records lack viewedAt and are excluded by the index query — they age out naturally as the user browses and new rich records fill in. Mentioned in tests.
• Snapshot counts for task groups: status counts may be partial if not all pages were fetched when the group was viewed — acceptable for a history snapshot.

Possible follow-up (out of scope)

A comment on the issue suggested a "my recent tasks" filtered by owner. That would require storing task.metadata.owner and filtering; noted here for future work.

Changes

• ui/src/utils/db.js: db.version(2) adds viewedAt secondary index
• ui/src/views/Tasks/ViewTask/index.jsx: updateTaskIdHistory stores name, source, taskQueueId, created, deadline, state, viewedAt
• ui/src/views/Tasks/TaskGroup/index.jsx: updateTaskGroupIdHistory stores decision-task metadata + statusCount + viewedAt
• ui/src/views/Tasks/NoTask/index.jsx: queries by viewedAt, renders rich list
• ui/src/views/Tasks/NoTaskGroup/index.jsx: queries by viewedAt, renders rich list with status counts
• Tests added for both display components (jest.mock for IndexedDB, covers rich + legacy + empty states)

Fixes #8711