Add welcome modal with bridge and getting started links

[wangxq-cc]

Closes #13.

Changes:

• Adds a first-run welcome modal persisted through userSettings.hasSeenWelcome.
• Adds concise onboarding steps for wallet connection, Arbitrum bridging, and dashboard entry points.
• Adds direct Arbitrum Bridge, Trading Docs, and Pool Docs links.
• Adds a persistent Help navigation item so users can reopen the guide later.

Validation:

• npm ci
• npm run build

Build passes. The build still reports existing Svelte/Rollup warnings unrelated to this change.

[wangxq-cc]

Note on CI: the Vercel status currently reports Account is blocked, which appears unrelated to this code change. I validated the branch locally with npm run build.