fix(m7): surface evidence whitelist in UI + filter the OS file picker

Reported by the user: the blue-side dropzone said "≤ 25 MB each" but
nowhere did it list the accepted extensions, and the OS file picker
showed "All files" — so an operator could spend the time picking a
`.exe` only to get a 400 back.

- New constants `EVIDENCE_ALLOWED_EXTENSIONS` + `EVIDENCE_MAX_BYTES` in
  `lib/missions.ts`. Manual mirror of the backend whitelist (commented
  cross-reference). One source of truth on the client.
- Dropzone now prints `Accepted: .png · .jpg · .jpeg · .pdf · .txt ·
  .log · .json · .csv · .evtx · .zip · max 25 MB / file`
  (testid `evidence-allowed-formats`).
- File input gains `accept=".png,.jpg,..."` so the OS picker pre-filters
  to those extensions instead of "All files".
- `handleFiles` rejects drag-and-drops of unsupported extensions on the
  client too (still re-checked server-side — defence in depth, not a
  security boundary).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

frontend/src/pages/MissionTestPage.tsx

@@ -38,6 +38,8 @@ import {
 } from '@/lib/api';
 import { useAuth } from '@/lib/auth';
 import {
+  EVIDENCE_ALLOWED_EXTENSIONS,
+  EVIDENCE_MAX_BYTES,
   MISSION_TEST_STATE_ACCENT,
   MISSION_TEST_STATE_LABEL,
   VALID_TEST_TRANSITIONS,
@@ -403,10 +405,19 @@ function BlueZone({ test, missionId, canWriteBlue, detectionLevels }: BlueZonePr
   function handleFiles(files: FileList | null) {
     if (!files || files.length === 0) return;
     setDropError(null);
-    // Per spec §M7, max 25 MB per file is server-enforced; this is just a
-    // friendly UX guardrail so the user does not waste a roundtrip.
+    // Per spec §M7, max 25 MB + ext whitelist are server-enforced; the client
+    // checks too so a drag-and-drop of a `.exe` doesn't waste a roundtrip
+    // and the operator gets immediate feedback.
     for (const f of Array.from(files)) {
-      if (f.size > 25 * 1024 * 1024) {
+      const dotIdx = f.name.lastIndexOf('.');
+      const ext = dotIdx >= 0 ? f.name.slice(dotIdx).toLowerCase() : '';
+      if (!ext || !(EVIDENCE_ALLOWED_EXTENSIONS as readonly string[]).includes(ext)) {
+        setDropError(
+          `"${f.name}" has an unsupported extension. Accepted: ${EVIDENCE_ALLOWED_EXTENSIONS.join(', ')}.`,
+        );
+        continue;
+      }
+      if (f.size > EVIDENCE_MAX_BYTES) {
         setDropError(`"${f.name}" exceeds the 25 MB limit.`);
         continue;
       }
@@ -494,13 +505,27 @@ function BlueZone({ test, missionId, canWriteBlue, detectionLevels }: BlueZonePr
         >
           <p className="font-mono text-2xs text-text-dim">
             {canWriteBlue
-              ? 'Drag files here or click to pick (≤ 25 MB each)'
+              ? 'Drag files here or click to pick'
               : 'Read-only — no upload permission'}
           </p>
+          {canWriteBlue && (
+            <p
+              className="mt-1 font-mono text-3xs uppercase tracking-wider2 text-text-dim"
+              data-testid="evidence-allowed-formats"
+            >
+              Accepted: {EVIDENCE_ALLOWED_EXTENSIONS.join(' · ')} ·{' '}
+              max 25 MB / file
+            </p>
+          )}
           <input
             ref={fileInputRef}
             type="file"
             multiple
+            // `accept` is the native filter the OS file-picker honours. It
+            // accepts a comma-separated list of extensions or MIME types.
+            // Servers re-check both, so this is a UX nicety, not a security
+            // boundary.
+            accept={EVIDENCE_ALLOWED_EXTENSIONS.join(',')}
             className="hidden"
             onChange={(e) => handleFiles(e.target.files)}
             data-testid="evidence-file-input"