feat: add a guided tour to the Analytics page

[mikewheeleer]

Summary

Context: while working this issue I found this repo has a large amount of previously-deleted UI infrastructure (a single upstream commit removed ~2,500 files / 170K lines, including the app shell, command palette, and several modals -- none have been rebuilt since). Per maintainer guidance I received, I'm building small, fresh, self-contained pieces rather than trying to resurrect the old deleted implementations wholesale.

The create wizard (src/app/create/page.tsx) has a guided tour via useGuidedTour, but no other page does. That hook can't be reused as-is: it's tightly coupled to the wizard's 3-step model (TOUR_STEPS is hardcoded to wizardStep: 1|2|3, and stepping through the tour actively drives setWizardStep).

What this adds

• usePageTour (src/hooks/usePageTour.ts) -- a small, page-agnostic tour hook. Tracks a step index over a flat PageTourStep[] list and persists "seen" state to localStorage under a caller-supplied key, so any page can have its own independent tour.
• GuidedTour (src/components/onboarding/GuidedTour.tsx) -- the tour dialog UI, which didn't exist at all before this PR (it's imported by create/page.tsx but was one of the files deleted in the incident above). Built on the existing Dialog primitive (focus trap, Escape-to-close, background inert, portal) instead of a viewport-anchored tooltip -- reuses already-tested accessibility scaffolding rather than adding new custom positioning/z-index logic. A "Show me on the page" link scrolls the step's target element into view instead.
• Wires a 3-step tour into the Analytics page (view toggle → KPI cards → trend charts) behind a "Take a tour" button in the header.

Testing

• usePageTour.test.ts -- 8 tests (start/end, next/prev with clamping, empty-steps no-op, per-key localStorage persistence).
• GuidedTour.test.tsx -- 9 tests (renders nothing when inactive/no step, progress text, Back button hidden on step 1, Next→Finish label on last step, Skip/Next/Back callbacks, scroll-into-view).
• All 17 new tests pass.

Note on verification: this page -- and most of the repo -- currently doesn't build (pnpm build fails on a pre-existing, unrelated missing module, @/components/KPICard, part of the same deletion incident). I confirmed via git stash A/B comparison that this diff adds zero new build errors and zero new test failures (baseline: 22 failed test files / 37 failed tests / 488 passed, unchanged except my +17 passing). Also removed one unused local (isAnyLoading) in the same file -- the pre-commit lint hook flagged this pre-existing dead code on a file I was already touching; one-line removal, no behavior change.

Closes #924

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