fix: preserve Ctrl+C copy behaviour when text is selected

Resolves #195. Before sending an interrupt signal, check whether
the user has text selected. If so, let the browser handle Ctrl+C
as a native copy rather than interrupting Claude.

src/routes/+page.svelte

@@ -240,9 +240,10 @@
       return;
     }
 
-    // Ctrl+C - Interrupt (only when processing)
+    // Ctrl+C - Interrupt (only when processing AND no text is selected)
     if (event.ctrlKey && event.key === "c") {
-      if (get(isClaudeProcessing)) {
+      const hasSelection = Boolean(window.getSelection()?.toString());
+      if (get(isClaudeProcessing) && !hasSelection) {
         event.preventDefault();
         handleInterrupt();
         return;

src/routes/page.test.ts

@@ -0,0 +1,38 @@
+/**
+ * +page.svelte keyboard shortcut tests
+ *
+ * Tests the pure decision logic for the Ctrl+C keyboard shortcut handler.
+ * The handler should only intercept Ctrl+C (to send an interrupt signal) when:
+ *   - Claude is currently processing a request, AND
+ *   - No text is currently selected (so normal copy behaviour is preserved)
+ *
+ * Manual testing checklist:
+ * - [ ] Ctrl+C with text selected copies the text (browser default)
+ * - [ ] Ctrl+C with no text selected and Claude processing sends an interrupt
+ * - [ ] Ctrl+C with no text selected and Claude idle does nothing special
+ */
+
+import { describe, it, expect } from "vitest";
+
+// Mirror the Ctrl+C interrupt decision logic from +page.svelte
+function shouldInterruptOnCtrlC(isProcessing: boolean, hasSelection: boolean): boolean {
+  return isProcessing && !hasSelection;
+}
+
+describe("Ctrl+C interrupt logic", () => {
+  it("interrupts when Claude is processing and no text is selected", () => {
+    expect(shouldInterruptOnCtrlC(true, false)).toBe(true);
+  });
+
+  it("does not interrupt when text is selected, even if Claude is processing", () => {
+    expect(shouldInterruptOnCtrlC(true, true)).toBe(false);
+  });
+
+  it("does not interrupt when Claude is idle and no text is selected", () => {
+    expect(shouldInterruptOnCtrlC(false, false)).toBe(false);
+  });
+
+  it("does not interrupt when Claude is idle and text is selected", () => {
+    expect(shouldInterruptOnCtrlC(false, true)).toBe(false);
+  });
+});