fix: add per-page metadata to dashboard, download, and community pages

[bokiko]

Problem

All three main pages (/dashboard, /download, /community) had "use client" at the top level. In Next.js App Router, client components cannot export metadata — so every page fell back to the generic layout title/description. The template: "%s | PingDiff" in layout.tsx was dead code for these routes.

Concrete impact:

• Sharing any page on Discord, Twitter, or iMessage showed the homepage title and description
• Browser tab always read "PingDiff - Test Your Game Server Connection" regardless of which page you were on
• Search engines indexed all three pages with duplicate meta descriptions

Fix

Split each page into two files following the standard App Router pattern:

File	Role
page.tsx	Server component — exports metadata, renders the client component
*Client.tsx	Client component — all interactive state (useState, useEffect, etc.)

No logic was changed; this is a pure structural refactor.

Result

Route	Title	Description
/dashboard	Dashboard | PingDiff	View your ping test results, packet loss trends...
/download	Download | PingDiff	Download PingDiff for Windows — free, open source...
/community	Community | PingDiff	Share connection tips, compare ping results...

Each page also gets its own og:title and og:description for correct social previews.

Verification

• npm run build — passes with no errors
• eslint — no lint errors introduced