fix(person-mention): respect modified-click and middle-click for new-tab nav

Felix #7: handleMentionClick unconditionally preventDefault'd and goto'd,
breaking ctrl-click / cmd-click / shift-click / alt-click / middle-click —
"open in new tab" is a real workflow for researchers comparing two persons.

Add isPlainPrimaryClick() guard. Modified clicks fall through to the
browser's default anchor handling (the <a href="/persons/{id}"> opens in
the new tab as expected). Plain left-clicks still SPA-navigate via goto().

Three new tests assert ctrl-click, meta-click, and middle-click are not
preventDefault'd.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

frontend/src/lib/components/TranscriptionReadView.svelte.test.ts

@@ -370,6 +370,38 @@ describe('TranscriptionReadView — person-mention rendering', () => {
 		expect(card).toBeNull();
 	});
 
+	it('lets ctrl-click and meta-click fall through so users can open in a new tab', async () => {
+		render(TranscriptionReadView, {
+			blocks: [mentionBlock],
+			onParagraphClick: () => {}
+		});
+
+		const link = document.querySelector('a.person-mention')! as HTMLAnchorElement;
+
+		// ctrl-click (Linux/Win "open in new tab")
+		const ctrlClick = new MouseEvent('click', { bubbles: true, cancelable: true, ctrlKey: true });
+		const ctrlPrevented = !link.dispatchEvent(ctrlClick);
+		expect(ctrlPrevented).toBe(false);
+
+		// meta-click (macOS "open in new tab")
+		const metaClick = new MouseEvent('click', { bubbles: true, cancelable: true, metaKey: true });
+		const metaPrevented = !link.dispatchEvent(metaClick);
+		expect(metaPrevented).toBe(false);
+	});
+
+	it('lets middle-click fall through so users can open in a background tab', async () => {
+		render(TranscriptionReadView, {
+			blocks: [mentionBlock],
+			onParagraphClick: () => {}
+		});
+
+		const link = document.querySelector('a.person-mention')! as HTMLAnchorElement;
+		// button === 1 is middle mouse button
+		const middleClick = new MouseEvent('click', { bubbles: true, cancelable: true, button: 1 });
+		const prevented = !link.dispatchEvent(middleClick);
+		expect(prevented).toBe(false);
+	});
+
 	it('degrades to plain unlinked text when the person fetch returns 404', async () => {
 		vi.stubGlobal('fetch', vi.fn().mockResolvedValue({ status: 404, ok: false, json: vi.fn() }));