feat: add stream metadata support (name, category, memo)

[Vera3289]

Summary

Adds optional metadata (name, category, memo) to streams in both the contract and frontend.

Changes

Contract (contracts/streaming/src/lib.rs)

• Added StreamMetadata struct with name, category, memo string fields
• Added metadata: Option<StreamMetadata> to Stream and CreateStreamParams
• create_stream stores metadata separately if provided
• New update_stream_metadata function (sender-only auth)
• New get_stream_metadata query function

Types (types/stream.ts)

• Added StreamMetadata interface
• Added metadata?: StreamMetadata to StreamData and CreateStreamInput

Create form (app/app/create/page.tsx)

• Added optional Name, Category (select), and Memo fields, wired into CreateStreamInput.metadata

Stream cards (components/streams/stream-card.tsx)

• Header shows stream.metadata.name when set, otherwise falls back to direction label
• USD equivalent shown below token amount

Acceptance criteria

• StreamMetadata struct added to contract
• create_stream accepts optional metadata
• update_stream_metadata function for sender to edit
• Frontend create form has optional metadata fields
• Stream cards display name if set

Closes #166

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