Desktop app: file cards and file-preview panel can't reliably show an existing file

**Summary**

Two related, recurring faults in the Claude desktop app's file-delivery paths:

1. File cards sent into chat do nothing useful when clicked — no working preview, no open, no download.
2. The file-preview panel reports "Couldn't find this file" for a path that demonstrably exists — its own "Show in Explorer" button then finds it immediately.

**Environment**

Windows 11. Recurred across an app update: `1.40609.1.0` (3 Sept 2026) → `1.46388.4.0` (8 Sept 2026). Not a one-build regression.

**Evidence**

3 Sept and again 8 Sept: a file referenced by Claude Code (confirmed present via `ls`, correct size, tracked in git) triggered "Couldn't find this file" in the panel. "Show in Explorer" on that same error screen located it immediately — proving the panel's own fallback disagrees with its main lookup.

**Impact**

Real, repeated time lost with no in-app recovery beyond "Show in Explorer," which is itself only offered after the false error, not from the file-card path.

**Suggested fix**

Make "Show in Explorer" use the same path-resolution code as the primary preview load, so the two can't disagree about whether a file exists. Add a working open/preview action to file cards, or make clearly non-interactive if intentional.

No files or tests are named. Locate the desktop file-card and preview-panel entry points, then compare their path lookup with the Show in Explorer path. Reproduce on Windows 11 with an existing file; done means previews no longer report a false missing file and cards provide a working preview/open/download action or are clearly non-interactive.

索引モデルが issue の本文から書いたものです。