fix(canvas): name the canvas before creating its generation task

[k11kirky]

Problem

Every canvas generation task kicked off from the channel composer's Canvas mode (#3348) gets the title Generate canvas "Untitled canvas", and that placeholder also shows as the first prompt bubble before the agent starts. The composer creates the dashboard with the placeholder name at submit time, and auto-naming only ran after the task was created — renaming the dashboard but never the task, its backend description, or the generation prompt's header.

Changes

• useGenerateFreeformCanvas: for a still-unnamed canvas, generate the name from the instruction before creating the task (concurrently with model resolution, so the common path adds no latency). The real name now feeds the task title/description, the prompt header, the channel filing, and the completion toast; the dashboard rename is fired best-effort. Falls back to the placeholder if name generation fails, as before.
• Removed the now-unused post-task rename block and the tracker store's updateName.

How did you test this?

• pnpm --filter @posthog/ui typecheck — clean
• biome check on the changed files — clean
• pnpm --filter @posthog/ui test — 164 files / 1458 tests passed

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 75f04da.}