feat: add copy functionality for ai responses with Dyad tag formatting (#1290) (#1315)

## Summary
Adds AI response copy functionality to chat messages that preserves
formatting and converts Dyad-specific tags to clean, readable markdown.

## Changes
- **New `useCopyToClipboard` hook**: Parses Dyad tags and converts them
to professional markdown format
- **Updated `ChatMessage` component**: Positions copy button on left
side of approval status
- **Dyad tag conversion**: Transforms custom tags to readable format:
  - `<dyad-write>` → `### File: path/to/file.js` + code block
  - `<dyad-edit>` → `### Edit: path/to/file.js` + code block  
  - `<dyad-execute-sql>` → `### Execute SQL` + ```sql block
  - `<think>` → `### Thinking` + content

## Features
- ✅ Automatic programming language detection from file extensions  
- ✅ Professional markdown formatting with proper headings and code
blocks
- ✅ Tooltip showing "Copied" confirmation
- ✅ Reuses existing DyadMarkdownParser logic for consistency

closes (#1290)
    
<!-- This is an auto-generated description by cubic. -->
---

## Summary by cubic
Adds a Copy button to assistant messages that copies a clean Markdown
version of the response by converting Dyad tags and preserving code
blocks. Improves shareability and removes Dyad-only markup; addresses
Linear #1290.

- **New Features**
- Added useCopyToClipboard hook that parses Dyad tags to Markdown,
auto-detects code language, and cleans spacing.
- Updated ChatMessage to show a Copy button (with Copy/Copied tooltip)
to the left of approval status; disabled while streaming.
- Tag conversions: think → "### Thinking"; dyad-write/edit → "###
File/Edit: path" + fenced code; dyad-execute-sql → "### Execute SQL" +
sql block; other Dyad tags map to concise headings; chat-summary/command
are omitted.
- Added e2e tests for clipboard copy, Dyad tag stripping/formatting, and
tooltip states.

<!-- End of auto-generated description by cubic. -->

e2e-tests/copy_chat.spec.ts

@@ -0,0 +1,71 @@
+import { test } from "./helpers/test_helper";
+import { expect } from "@playwright/test";
+
+test("copy message content - basic functionality", async ({ po }) => {
+  await po.setUp({ autoApprove: true });
+  await po.importApp("minimal");
+
+  await po.sendPrompt("[dump] Just say hello without creating any files");
+
+  await po.page
+    .context()
+    .grantPermissions(["clipboard-read", "clipboard-write"]);
+
+  const copyButton = po.page.getByTestId("copy-message-button").first();
+  await copyButton.click();
+
+  const clipboardContent = await po.page.evaluate(() =>
+    navigator.clipboard.readText(),
+  );
+
+  // Test that copy functionality works
+  expect(clipboardContent.length).toBeGreaterThan(0);
+  expect(clipboardContent).not.toContain("<dyad-");
+});
+
+test("copy message content - dyad-write conversion", async ({ po }) => {
+  await po.setUp({ autoApprove: true });
+  await po.importApp("minimal");
+
+  await po.sendPrompt(
+    "Create a simple React component in src/components/Button.tsx",
+  );
+
+  await po.page
+    .context()
+    .grantPermissions(["clipboard-read", "clipboard-write"]);
+
+  const copyButton = po.page.getByTestId("copy-message-button").first();
+  await copyButton.click();
+
+  const clipboardContent = await po.page.evaluate(() =>
+    navigator.clipboard.readText(),
+  );
+
+  // Should convert dyad-write to markdown format (flexible path matching)
+  expect(clipboardContent).toContain("### File:");
+  expect(clipboardContent).toContain("```");
+  expect(clipboardContent).not.toContain("<dyad-write");
+});
+
+test("copy button tooltip states", async ({ po }) => {
+  await po.setUp({ autoApprove: true });
+  await po.importApp("minimal");
+
+  await po.sendPrompt("Say hello");
+
+  const copyButton = po.page.getByTestId("copy-message-button").first();
+
+  // Check initial tooltip
+  await copyButton.hover();
+  const tooltip = po.page.locator('[role="tooltip"]');
+  await expect(tooltip).toHaveText("Copy");
+
+  // Copy and check "Copied!" state
+  await po.page
+    .context()
+    .grantPermissions(["clipboard-read", "clipboard-write"]);
+  await copyButton.click();
+  await copyButton.hover();
+  await expect(tooltip).toHaveText("Copied!");
+});