feat: gate onboarding behind hard sudoku puzzle

[posthog]

Summary

• Adds a hidden first onboarding step that shows Arto Inkala's hard sudoku puzzle. The user must place valid 1–9 digits across rows, columns, and 3x3 boxes before they can proceed.
• Gated by the new posthog-code-secret-sudoku-onboarding feature flag, set to 0% rollout for now. When the flag is off, the onboarding flow is unchanged.
• When the flag is on, the puzzle becomes a hard gate: arrow-key step navigation and the dev "Skip setup" button are disabled, and a persisted hasSolvedSecretSudoku flag forces the user back to the puzzle if they somehow land on a later step.

Implementation

• apps/code/src/shared/constants.ts — new SECRET_SUDOKU_ONBOARDING_FLAG constant.
• apps/code/src/renderer/features/onboarding/sudoku.ts — puzzle data + solver utilities (isBoardSolved, findConflicts).
• apps/code/src/renderer/features/onboarding/components/SudokuStep.tsx — interactive 9x9 grid (keyboard + click input, conflict highlighting, peer highlighting, on-screen number pad). The "Verify" button only fires onSolved when the board is correctly filled.
• apps/code/src/renderer/features/onboarding/types.ts — added "secret-sudoku" step.
• apps/code/src/renderer/features/onboarding/stores/onboardingStore.ts — persists hasSolvedSecretSudoku.
• apps/code/src/renderer/features/onboarding/hooks/useOnboardingFlow.ts — gate logic: prepends the puzzle to activeSteps and forces currentStep back to it whenever the gate is unresolved.
• apps/code/src/renderer/features/onboarding/components/OnboardingFlow.tsx — renders SudokuStep, disables arrow hotkeys + skip button while on the gate.

Test plan

• With the flag off, onboarding behaves exactly as before.
• With the flag on, the sudoku step is the first thing the user sees.
• Cell input via keyboard (1–9, Backspace) and click works; arrow keys move focus across the grid.
• Conflicting digits highlight in red; the Verify button stays disabled until the board is fully filled.
• Verify reports "not quite right" for valid-but-wrong fills, and advances when the board is solved.
• After solving, subsequent steps render normally and the gate does not reappear (verified via the persisted flag).
• Logging out resets hasSolvedSecretSudoku so the gate will reappear on the next session.

---

Created with PostHog Code