feat(web): wire Study Buddy into the reader (AI-038b)

[mrviduus]

Phase 6 AI-038, slice b — the panel is now reachable end-to-end: select a passage in the reader → "Help me understand this" → the agent investigates live (steps stream in, answer at the top).

Changes

• SelectionToolbar gains a "Help me understand this" action (sparkles icon) next to Explain, rendered only when onStudyBuddy is wired (catalog editions).
• ReaderHighlights passes the whole selected passage up via a new onStudyBuddy(passage) prop, then clears the selection.
• ReaderPage holds the passage state and renders StudyBuddyPanel (catalog editions only — same gate as Ask), threading the current chapter number so get_chapter/get_chapter_summary have context. Selecting a new passage re-runs the agent.
• i18n: reader.selectionToolbar.studyBuddy.

Verification

• tsc --noEmit + pnpm build clean; full web suite green (517 — the AI-038a hook/panel/api unit tests already cover the behaviour).
• Confirmed no e2e clicks the selection toolbar by positional index (the AI-026a top-bar lesson), so the new button can't shift another test's target.
• The live agent run (key + corpus) is verified on prod after deploy; closes the visible half of Phase 6's Study Buddy.

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