tui: " File " badge label invisible with the system theme

Description

With the system theme, the File label on file-attachment chips in user messages is invisible — it blends into the badge background, leaving a solid colored block (note the QUEUED chip below it renders fine):

Root cause

packages/tui/src/routes/session/index.tsx paints the label with fg: theme.background over bg: theme.secondary. Built-in themes define an opaque background, but the generated system theme (generateSystem() in packages/tui/src/theme/index.ts) deliberately makes it fully transparent to respect terminal transparency — so the label disappears into the badge. The QUEUED badge just below already handles this via selectedForeground(theme, color); the file badge doesn't.

Reproduction

1. Use the system theme.
2. @-mention a file in the prompt and send.
3. The chip in the submitted message is a solid colored block — no visible File label.

(reproduced on 1.18.18; still present on dev)