feat: Add recipient opt-in mechanism for incoming streams

[Austinaminu2]

Description

Currently, anyone can create a stream to any Stellar address without the recipient's consent. The recipient has no way to reject an unwanted stream. This can lead to:

• Cluttered dashboards with unsolicited streams
• Potential regulatory issues (receiving unsolicited tokens)
• Poor UX when unknown senders create streams

Proposed Solution

Option A: Frontend-only (simpler)

• Add a "Hide stream" feature so recipients can dismiss unwanted streams from their dashboard
• Hidden streams stored in localStorage, can be un-hidden
• No contract changes needed

Option B: Contract-level (more robust)

• Add an accept_stream() function — stream starts unlocking only after recipient accepts
• Unaccepted streams can be cancelled by the sender with full refund
• Adds a pending status to streams

Recommend starting with Option A for immediate UX improvement, with Option B as a follow-up.

Acceptance Criteria (Option A)

• "Hide" button on stream cards for received streams
• Hidden streams don't appear in dashboard or counts
• "Show hidden streams" toggle to view/un-hide
• Hidden stream IDs persisted in localStorage
• "Block sender" option to auto-hide future streams from an address