fix(chat): show send button for attachment-only sends (#956) (#956)

The AnimatedContent gate on the composer's send button required
inputFieldValue.text.isNotBlank() in addition to canSend, so attaching an
image or file with an empty input hid the button entirely — the attachment
could never be sent on its own. ChatInputPolicy.canSend already allowed
attachment-only sends and ChatViewModel.sendMessage already accepted blank
text with pending attachments, but the button never rendered to trigger it.

Drop the stale text gate so the send affordance tracks canSend (non-empty
text OR pending attachments, plus connection). The backend handles
image-only turns natively: prompt.submit with empty text + queued images
routes through build_native_content_parts, which wraps the pixels with an
[Image attached at: ...] hint.

Add a Compose UI regression test asserting the send button is displayed
with a pending image attachment and an empty input.

Co-authored-by: M57 <hy4ri@users.noreply.github.com>

app/src/main/java/com/m57/hermescontrol/ui/chat/components/ChatComposer.kt

@@ -278,9 +278,12 @@ fun ChatInputBar(
                                         innerTextField()
                                     }
 
-                                    // Send button INSIDE the field
+                                    // Send button INSIDE the field. Shown whenever a send is
+                                    // possible — text typed OR an attachment pending (issue
+                                    // #956): the old text-only gate hid the button entirely
+                                    // for attachment-only sends.
                                     AnimatedContent(
-                                        targetState = canSend && inputFieldValue.text.isNotBlank(),
+                                        targetState = canSend,
                                         transitionSpec = {
                                             (scaleIn(initialScale = 0.8f) + fadeIn())
                                                 .togetherWith(scaleOut(targetScale = 0.8f) + fadeOut())