feat(I6): FE/BE enum contract test — pin JS mirrors to backend canon

The plain-JS frontend re-declares two backend enum value-sets by hand:
download-event statuses (downloadStatus.js) and platform keys
(platformColor.js). With no TS codegen, drift is silent — the same class
as the last_verified_at field mismatch.

tests/test_fe_be_contract.py parses both JS mirrors and asserts they equal
the backend canon (downloads._ALLOWED_STATUSES, platforms.known_platform_keys())
so a change on either side fails CI on whichever moved. Backend is the
source of truth. Documented the invariant in both JS file headers.

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

frontend/src/utils/downloadStatus.js

@@ -1,7 +1,8 @@
 // Single source of truth for the download-event status enum -> display
 // metadata (label/color/icon). Mirrors the backend ENUM
 // (pending|running|ok|error|skipped). Used by the stat chips, the filter
-// dropdown, and the event rows so they never drift apart.
+// dropdown, and the event rows so they never drift apart. The value set is
+// pinned against backend _ALLOWED_STATUSES by tests/test_fe_be_contract.py.
 
 export const DOWNLOAD_STATUSES = [
   { value: 'pending', label: 'Queued',    color: 'grey',    icon: 'mdi-clock-outline' },

frontend/src/utils/platformColor.js

@@ -1,7 +1,9 @@
 // Single source of truth for platform → color + icon mapping. Used by
 // PlatformChip and any other GS-style platform-tagged surface. The six
 // platforms FC supports map 1:1 to the GS palette; unknown platforms fall
-// back to grey + mdi-web. Operator-confirmed scope 2026-05-27.
+// back to grey + mdi-web. Operator-confirmed scope 2026-05-27. The ICONS key
+// set is pinned against backend known_platform_keys() by
+// tests/test_fe_be_contract.py.
 
 const ICONS = {
   patreon:       'mdi-patreon',