feat(canvas): add recent tasks tab to Activity view

[raquelmsmith]

Problem

The Activity view only surfaced channel mentions. It should also let you see the tasks you've recently worked on, without leaving the Activity surface.

Why: Raquel asked for the Activity view to show a paged list of her recent tasks alongside the existing notifications, with each item showing the task name and its current-status icon.

Changes

• Added a Mentions / Tasks tab switcher to the Activity view. Mentions is unchanged; Tasks is new.
• The Tasks tab lists the current user's tasks, newest first. Each row reuses the sidebar's TaskIcon for the live status icon plus the task title and a relative timestamp; clicking opens the task (in its channel when it's filed to one).
• The list is offset-paginated (new getTasksPage API method + useRecentTasksInfinite hook) and auto-loads the next page from an IntersectionObserver sentinel as you scroll near the bottom, with a spinner while fetching and an end-of-list marker.
• Reuses existing primitives throughout (quill Empty/Spinner/Button, useInView, TaskIcon) and the existing open_task channel analytics event.

How did you test this?

• pnpm --filter @posthog/api-client typecheck and pnpm --filter @posthog/ui typecheck — clean.
• Biome check on all changed files — clean.
• pnpm --filter @posthog/ui test — all 1566 tests pass.
• Not yet exercised in the running app; pulling down for a manual pass is welcome.

Automatic notifications

• Publish to changelog?
• Alert Sales and Marketing teams?

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Created with PostHog Code

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[github-actions]

React Doctor found no issues in the changed files. 🎉

_{Reviewed by React Doctor for commit e6bea6c.}