Implement dispute evidence management UI

[0x-fyve]

Overview

Implements the dispute evidence management workflow for dispute detail pages, replacing simulated local submissions with authenticated API-backed uploads, previews, downloads, and status tracking.

Related Issue

Closes #183

Changes

• Added authenticated dispute access checks so only admins or contract participants can view and manage dispute evidence.
• Added evidence listing, multipart evidence submission, encrypted file storage metadata, and authorized evidence preview/download endpoints.
• Updated the dispute detail UI to load live dispute/evidence data, submit notes/files, show upload progress, remove queued files, and display a chronological evidence record.
• Added dispute evidence validation constants and schema columns for encrypted evidence metadata.
• Added focused API and dashboard tests for access control, validation, storage metadata, downloads, and UI loading.

Verification Results

• git diff --check - passed
• npm.cmd run lint - passed with 4 pre-existing warnings in unrelated files
• npm.cmd test -- app/dashboard/disputes/[id]/page.test.tsx __tests__/api/dispute-evidence.test.ts - passed, 2 files / 6 tests
• npm.cmd run build - passed

Note: the full test suite was also attempted during local verification and still fails in existing deliverables/reviews tests unrelated to this change.

Acceptance Criteria

Requirement	Status
Real evidence submission instead of simulated UI behavior	Done
Validate uploaded evidence files	Done
Store evidence securely with metadata and hash	Done
Restrict evidence access to authorized dispute participants/admins	Done
Show submitted evidence and dispute progress dynamically	Done
Add focused automated coverage	Done

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