test: add Vitest client-side test suite + CI

[shimoncohen]

Summary

• Adds a Vitest + @testing-library/svelte (jsdom) harness for the client-side code (no server/S3/E2E scope).
• 28 tests across 4 files: demoCache storage util, demoIndex JTD schema, flems.svelte editor wrapper, and the demo +page.svelte (cache banner, debounced save, clear-confirm, route-change reload).
• +page.svelte is tested by mocking the Flems child + $app/stores + $env/dynamic/public; the real editor is never mounted.
• Adds a test job to pull-request.yaml so the suite runs on every PR.

Notes

• Pins @testing-library/svelte@^4 and vitest@^1 — v5 of testing-library needs Svelte 5; this repo is Svelte 3.59. The v5-only svelteTesting() plugin is replaced by resolve.conditions: ['browser'] + the @testing-library/svelte/vitest setup entry.
• SvelteKit virtual modules ($app/stores, $env/dynamic/public) are aliased to inert stubs so Vite can transform components under the plain svelte() plugin; per-test vi.doMock supplies the real behavior.
• Tests characterize the existing cache behavior, including two intentional drifts from the original cache design doc: saveCache stores a bare JSON array (not { files, savedAt }), and debounceMs = 0 is accepted (only NaN/negative fall back to 500).

Test Plan

• npm run test → 28/28 pass
• npm run check (svelte-check) → 0 errors
• CI Run Vitest job green on this PR

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