Fix verified image uploads and failed-panel retries

[Manho]

Summary

• add provider-specific, preview-verified image uploads for Grok, DeepSeek, and Kimi
• wait for per-panel Fill acknowledgements before reporting success or clearing the shared payload
• retain text and attachments after partial failures, then retry only failed panels
• reconcile delayed Kimi uploads before retrying to avoid duplicate attachments or text
• recognize Qwen China uploads completed through its native drop path
• report complete Fill failures with an unambiguous panel count

Validation

• npx vitest run --exclude='tests/e2e/**' (38 files, 336 tests)
• node --check for all changed JavaScript files
• git diff --check origin/main...HEAD
• verified real image previews in macOS Chrome across the supported providers
• verified a partial failure retains the shared payload and a successful retry clears it
• verified Retry targets only the failed panel and carries retry: true
• verified a delayed Kimi upload is reconciled on retry without adding a duplicate image or text
• verified Qwen China reports success only after its drop-created preview appears

Refs #52