feat(evidence): inline preview for images and text/JSON files

Adds a View button (eye icon) on each evidence card for previewable file
types. Opens a full-screen modal:
- Images (png/jpg/gif/webp/svg/…): rendered directly via <img> tag
- JSON: fetched authenticated, pretty-printed in green mono
- Text/log/md/csv/xml/yaml/…: fetched authenticated, shown in <pre>

Non-previewable files only show the Download button as before.
Modal closes on Escape or backdrop click.

frontend/src/api/evidence.ts

@@ -63,6 +63,21 @@ export async function getEvidence(evidenceId: string): Promise<EvidenceOut> {
   return data;
 }
 
+// ── Content fetch (for inline preview) ────────────────────────────
+
+/** Fetch raw file content for inline preview (text / JSON). */
+export async function getEvidenceRawContent(
+  evidenceId: string,
+): Promise<{ text: string; contentType: string }> {
+  const response = await client.get(`/evidence/${evidenceId}/file`, {
+    responseType: "blob",
+  });
+  const blob = response.data as Blob;
+  const text = await blob.text();
+  const contentType = (response.headers["content-type"] as string) || "";
+  return { text, contentType };
+}
+
 // ── Delete ─────────────────────────────────────────────────────────
 
 /** Delete an evidence record (only in editable states). */