Desktop app: show full filename on hover for truncated file attachment cards

**Environment:** Claude desktop app (Code tab), macOS.

**Problem:** When a session has file attachments with long names (e.g. build artifacts like `foundational-m365-agent-staging-1.0.0-7b97c24605.zip`), the file-card UI truncates the name to fit the box. There is currently no way to see the full filename without downloading the file — hovering shows nothing.

When several attachments share a long common prefix and differ only near the end (versioned builds, per-environment artifacts like `...-staging-...` vs `...-prod-...`), the truncated cards become indistinguishable, and it's easy to grab the wrong file.

**Suggestion:** On hover over a file card, show the full filename — a native `title` tooltip would already solve it; a styled tooltip or middle-ellipsis truncation (keeping the distinctive tail visible) would be even better.

**Why it matters:** Sessions that produce versioned deliverables routinely attach files whose distinguishing suffix (version, SHA, environment) is exactly the part that gets truncated.

Start in the Claude desktop app's Code tab and locate the attachment file-card UI; inspect how long filenames are rendered and how hover behavior is handled. Test several long filenames with shared prefixes, and consider macOS behavior; done means hovering reveals the complete filename without downloading it.

Written by the indexing model from the issue text.