[Bug] Composer attachment remove button not reachable by keyboard (a11y)

**Describe the bug**
The "remove" (×) button on message composer attachments (both the generic
file chip and the image/video thumbnail) only appears on mouse hover
(`hidden group-hover:flex` in ComposerAttachments.tsx). Since the button is
`display: none` until hover, it's removed from the tab order entirely —
there's no way to reach or activate it with the keyboard.

**To Reproduce:**
1. Open the message composer and attach a file (or image).
2. Tab through the composer without touching the mouse.
3. Try to remove the attachment using only the keyboard.

**Expected behavior:**
The remove button should be reachable via keyboard, e.g. by also showing it
on focus-within (`group-focus-within:flex`), not just on hover.

**Supporting Material**
Screenshot attached — attachment chip with remove button only visible via
hover.

**Environment:**
- Desktop app (Tauri)

**Additional context**
Compare with the upload-cancel button in the same file
(`ComposerAttachments.tsx`, upload placeholder), which is always visible/focusable
and doesn't have this issue.