upgrade: date package upgrade for Solid 2.0

[davedbase]

Bumps peer dependencies to solid-js@^2.0.0-beta.10 and @solidjs/web@^2.0.0-beta.10

• Drops @solid-primitives/timer and @solid-primitives/memo deps; inlines TimeoutSource type
• createDateNow — converts createEffect to the required split (compute, apply) two-argument form
• createCountdown — replaces removed createComputed with createRenderEffect(compute, apply)
• createDate — replaces removed createWritableMemo with createSignal(fn) writable-computed overload
• createStore import moved from solid-js/store to solid-js; isServer import moved to @solidjs/web
• Tests updated: flush() after signal writes (pull-based reactivity requires explicit flush outside tracking contexts), signals created outside createRoot to avoid SIGNAL_WRITE_IN_OWNED_SCOPE
• New test/server.test.ts for SSR safety across all five primitives

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