feat(tarko): support real data file uploads

[SC123667]

Summary

• persist non-image chat attachments under uploads/ in the current Agent workspace through a new /api/v1/files/upload endpoint
• show uploaded-file chips, progress, removal, and errors in both welcome-page and session chat inputs while preserving the existing image data-URL flow
• append safe workspace-relative paths to the submitted message so the Agent can open and analyze CSV, JSON, documents, and other data files with its workspace tools
• reuse the existing CSRF flow for multipart requests, sanitize filenames, add collision-resistant suffixes, limit uploads to 10 files / 20 MB each, and avoid exposing absolute host paths
• add a patch changeset plus server and UI regression tests

Why

The file picker previously accepted only images, and the change handler silently ignored every non-image file. The UI could therefore look as if a data file had been selected even though no bytes reached the Agent workspace.

This implementation is based on current main and follows the current /api/v1 and CSRF conventions. It is also a current-main alternative related to #1557 while keeping image attachments on their existing client-side path.

Fixes #323.

Validation

• pnpm install --frozen-lockfile
• pnpm bootstrap
• pnpm test — 842 passed, 13 skipped
• pnpm --filter @tarko/agent-server test — 66 passed
• pnpm --filter @tarko/agent-ui test — 4 passed
• pnpm --filter @tarko/agent-server run build
• pnpm --filter @tarko/agent-ui run build
• Prettier check and git diff --check

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