refactor: consolidate stream validation logic into a shared utility w…

[emarkees]

Description

Resolves audit issue: [Audit] Three divergent implementations of "create a stream"

Previously, stream creation logic had three separate implementations across the codebase (StreamCreationWizard, CreateStreamContent, and DashboardView inline settings form). These entry points enforced divergent validation rules:

• Only two of three checked Stellar recipient public key format.
• Only one checked available wallet balance.
• Template management and draft persistence logic were duplicated independently.

Key Changes

• Unified Validation & Balance Enforcement: Integrated useStreamForm into DashboardView inline stream creation form (dashboard-view.tsx) and passed walletBalance to validateStreamForm. All three entry points now enforce identical validation rules (recipient Stellar public key format via isValidStellarPublicKey, token selection, positive amount, maximum precision of 7 decimal places, available wallet balance check, and valid duration).
• Refactored Draft Persistence: Updated saveDraft in useStreamForm.ts so pristine forms with empty recipient and amount fields do not save unwanted draft entries when default initial data is set.
• Unified Template Storage: Standardized template storage across wizard and dashboard using flowfi.stream.templates.v1.

Verification

• Unit Tests: Executed full Vitest test suite (39 test files, 367 tests passing, 0 failing).
• Linting: Passed npm run lint across the frontend workspace with 0 errors.

Closes #1263