Remove Error frequency tile from dashboard performance view

[SanAlexis]

Context

The dashboard performance view currently renders an Error frequency tile that shows wrong-answer rate as a flat percentage, with a placeholder "Complete 3 sessions to see your error trends." until enough data exists.

Source: components/dashboard/dashboard-performance-view.tsx#L228-L236

Decision

Remove the tile. Error analytics are already covered more meaningfully by #75 — Most frequent error types display on the profile page, which classifies errors by type (Phase 3 PersonalReflection.error_type) rather than reporting a single aggregate rate. A duplicate flat-rate tile on the dashboard adds noise without insight and adds a third "placeholder until N sessions" state to the dashboard.

Scope

Delete the tile and every code path that feeds it:

• components/dashboard/dashboard-performance-view.tsx
  • Remove the third <section> (the Error frequency block)
  • Remove errorRate from DashboardPerformanceViewProps
  • Remove errorTitle and errorAfterThreeSessions from the labels type and destructuring
  • Remove the AlertIcon import if it becomes unused
• app/[locale]/dashboard/page.tsx
  • Drop errorRate and the two error labels from the component passthrough
• lib/demo/data.ts
  • Remove the errorRate computation and the field from the metrics return type / consumers
• messages/en.json and messages/fr.json
  • Remove keys: errorTitle, errorAfterThreeSessions, errorFrequencyHint

Acceptance

• Dashboard no longer renders the Error frequency tile
• No dead references to errorRate, errorTitle, errorAfterThreeSessions, or errorFrequencyHint anywhere in the codebase
• Type-check and build pass
• Visual check on the dashboard in both en and fr locales