feat(frontend): Phase 3.3 — System Prompt Configuration page
- SystemPromptsPage: profile selector, activation, edit with TanStack Query
- ProfileList: 3 profile cards (A/B/C) with active indicator + edit button
- PromptEditor: 3 monospace textareas, placeholder badges, char count,
unknown placeholder warnings, per-step reset (↺), action bar
- PlaceholderDocs: info box showing {question}/{chunks}/{context}
- Data layer: +7 types, +6 API functions, +6 TanStack Query hooks
- Routing: /system-prompts route + NavBar link
- Tests: 27 tests (PlaceholderDocs 6, ProfileList 7, PromptEditor 14)
- 0TS errors, 27/27 tests pass, 1 pre-existing e2e failure (unrelated)
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