ASA settings: link to Apple's OAuth setup docs

[bootuz]

What

In the Apple Search Ads API section of the Settings panel (web/src/components/SettingsPanel.svelte):

• Add a link to Apple's Implementing OAuth for the Apple Search Ads API docs.
• Drop the (free, no budget required), then Settings → API → Create New Client aside since Apple's docs now cover the credential-generation steps in more detail.

After:

Used for keyword suggestions for any app. Create a Search Ads account at searchads.apple.com. See Apple's OAuth setup guide ↗ for the Client ID / Client Secret details.

Why

Following the OAuth wire-shape fix in #6, users who hit credential issues now have one authoritative source to consult. Apple's console UI for creating API clients changes occasionally — pointing at their docs keeps our help text from drifting stale, and the docs page itself covers the JWT generation steps we don't (and shouldn't) document inline.

Reviewer notes

• First <a href> in this file, so I introduced target="_blank" rel="noopener noreferrer" as the convention for any future external links here.
• Used the existing text-amber-400 hover:underline palette (matches the "Replace" button in the same form) rather than introducing a blue link color — keeps the new element in-palette.
• Used   between "guide" and the ↗ glyph so a line wrap can't orphan the arrow.

Verification

• npm run check — 0 errors, 0 warnings across 144 files
• Manually verified the rendered text and that the link opens Apple's docs in a new tab