fix: handle Unicode attachment filenames

[KateNedelina]

Summary

• Make attachment filename sanitization safe for UTF-8 names, including Cyrillic filenames.
• Add regression coverage for Unicode names, Unicode-safe truncation, and the fallback filename.
• Preserve UTF-8 characters when agent stdout/stderr is split across arbitrary byte-stream chunks.
• Preserve a useful upload error when the server or proxy returns an empty response body.

Root cause

sanitize_filename measured the sanitized filename with String::len() and then sliced it with a byte offset (clean[..50]). Rust strings are UTF-8, so a filename containing a multi-byte character could be sliced in the middle of that character. The resulting panic aborted the upload request and the browser surfaced only:

Failed to upload attachment:

This was reproduced with a filename containing Cyrillic text (4636__исполнитель_подтвердился...).

The same class of corruption was also present in streamed agent output. The backend decoded each
ReaderStream chunk independently with from_utf8_lossy. When a multi-byte Cyrillic character
was split between two chunks, both incomplete sequences were replaced with �, producing text
such as де��ствий in persisted assistant summaries.

Implementation

• Truncate sanitized stems by Unicode scalar values with .chars().take(...) instead of byte offsets.
• Keep the existing 50-character sanitized-stem limit and fallback behavior unchanged.
• Add focused unit tests in crates/services/src/services/file.rs.
• Add a stateful Utf8ChunkDecoder shared by local process streaming and Codex command-output normalization.
• Use the same decoder for ACP JSON sent to agent stdin, so user prompts and tool messages remain intact.
• Keep incomplete UTF-8 sequences between chunks and only emit replacement characters for genuinely invalid bytes.
• Add tests covering every byte split of a Russian/emoji string and invalid-byte handling.
• Centralize attachment upload error extraction in the web API client so JSON errors, plain-text errors, HTTP status text, and empty bodies are handled consistently for all three upload endpoints.

Validation

Passing locally:

• pnpm run format
• cargo test -p services filename (3 passed)
• cargo test -p utils utf8_chunk_decoder (2 passed)
• cargo check -p local-deployment
• cargo check -p executors
• pnpm run web-core:check
• pnpm --filter @vibe/local-web run lint
• pnpm --filter @vibe/ui run lint
• Prettier check for the changed web-core file
• git diff --cached --check

The local checkout cannot reproduce the full GitHub Actions environment: the host does not have the glib-2.0 development package required by the workspace/Tauri crates, and the local Node heap limit is insufficient for the full Vite production bundle at the final chunk-rendering stage. The latest upstream main Test workflow was already failing before this branch (including frontend and backend jobs); the changed packages themselves pass the focused checks above.

Review notes

The commit is intentionally limited to the attachment failure and does not include the unrelated pre-existing local changes in package.json or packages/local-web/vite.config.ts.

[KateNedelina]

CI note: GitHub has held the Test workflow for this fork-based PR in action_required. Approving a workflow run for a fork requires upstream repository administrator rights, so an upstream maintainer must approve the run before its checks can execute. The latest main Test workflow was already failing before this branch; I documented the locally reproducible baseline limitations in the PR description. No merge should occur without the required independent maintainer review.