feat(chat): add copy message to clipboard for assistant messages

[369795172]

Summary

Adds a Copy message option to the assistant message context menu (⋮ → Copy message).

• Tapping it copies all text parts of the message to the system clipboard
• Shows a brief "Copied" toast confirmation
• The new item appears before "Fork from here" so the most-used action is at the top

Changes

app/src/main/java/com/yage/opencode_client/ui/chat/ChatMessageContent.kt

• Add ContentCopy icon import
• Add ClipData, ClipboardManager, Context, Toast, LocalContext imports
• Add val context = LocalContext.current in MessageRow
• Add DropdownMenuItem("Copy message") as the first item in the context menu

Test plan

• Open a chat session with assistant responses
• Tap the ⋮ menu on any assistant message bubble
• Verify "Copy message" appears as the first menu item
• Tap "Copy message" → verify toast "Copied" appears
• Paste into any text field → verify full message text is pasted correctly
• Verify "Fork from here" still works as before

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