Add welcome modal for first-time users (resolves #13)

[OutstandingRetard]

Closes #13

What

Adds a one-time welcome modal that appears the first time a user opens the dashboard. It explains the four main features of CAP v4 (perpetuals, pools, bridge, staking) and links directly to the bridge/Deposit modal.

The modal is gated by a localStorage flag so returning users don't see it. Users can dismiss it with Got it or Don't show again.

Files

• src/components/modals/Welcome.svelte (new) — the modal component. Reuses the existing Modal.svelte and theme variables. No new dependencies.
• src/lib/utils.js — adds localStorageStore(key, defaultValue) helper, a tiny SSR-safe Svelte writable that auto-persists to localStorage. Returns a regular writable so callers can use $store as usual.
• src/components/layout/Modals.svelte — registers the new modal in the existing dispatcher alongside the other 14. No new pattern, no new file structure.

Why this approach

• Fits existing patterns: matches the Connect.svelte / Settings.svelte / Deposit.svelte style exactly. No new abstraction.
• No new deps: Svelte 3 already supports writable + subscribe. localStorage is a built-in.
• Theme-aware: reuses --primary, --text0, --layer1dot5, etc.
• One-time, non-intrusive: gated by cap_welcome_seen flag. Can be re-opened via showModal('Welcome') from anywhere — easy to wire to a help button later if you want.

Test

1. npm i && npm run dev
2. Open in incognito → welcome modal appears.
3. Click Got it → modal closes.
4. Reload → modal does NOT appear (gated).
5. localStorage.removeItem('cap_welcome_seen') in DevTools + reload → modal appears again.
6. showModal('Welcome') from the console → modal opens on demand.

Screenshots

Will add once maintainers confirm preferred size (520px width used; happy to adjust).

Bounty

I'm claiming the $500 bounty on this issue per the issue body rules. Comment on #13 requesting assignment was already posted.