fix(frontend): clarify creator wallet mock vs Horizon data + persist quest edit via drafts API

[limxiy]

Description

This PR addresses two issues assigned to the creator dashboard:

1. Creator wallet — clarify mock earnings vs Horizon (#266)

• Removed the fake ,150.02 mock earnings figure from the balance card
• Renamed the balance section to Horizon Balances showing only real on-chain XLM/USDC
• Disabled the Withdraw Funds button with a Coming soon badge and tooltip
• Removed the View Earnings button (no earnings API exists yet)
• Updated labels to make clear that all displayed data reflects real on-chain balances
• Disabled Withdraw in the side-panel Withdrawal Method section

2. Persist quest edit via draft API (#264)

• Replaced the \console.log\ stub with an actual \POST /missions/drafts\ API call
• Added success/error notification toasts using inline banner components
• On success, the user is redirected back to the quest detail page after a brief delay
• Errors are surfaced inline with clear messages
• Utilizes the existing SEP-10 auth flow via \creatorApiFetch\ with Bearer token

Changes

• frontend/src/app/creator/wallet/page.tsx — Removed mock earnings, restructured balance card, disabled withdraw
• frontend/src/app/creator/quests/[questId]/edit/page.tsx — Implemented draft save via API, added notifications

Closes #266
Closes #264

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