fix(downloads): don't classify per-item download 404s as source-level Not Found

gallery-dl logs `[download][error] Failed to download <file>.part` when a
single post attachment's media URL expires or is deleted, then recovers
and proceeds with the rest of the run. Our error categorizer was letting
a urllib3 debug line like `"HEAD /media-u/v3/<id> HTTP/1.1" 404 0` match
the NOT_FOUND_PATTERNS list, flagging the whole source as deleted even
though gallery-dl kept downloading subsequent items successfully.

Adds "failed to download" to the per-item allowlist so the mask flips
when skip activity is present, matching the shape of the campaign-ID
fix. Total-failure runs still classify as failures.

Also renames the Download Details modal's "Error Log" panel to
"Verbose Log (stderr)" — gallery-dl's `-v` flag sends all logging
(including [debug] lines) to stderr, so the original label was
misleading. Drops the blanket red text color for the same reason.

frontend/src/views/Downloads.vue

@@ -334,9 +334,9 @@
               </v-expansion-panel-text>
             </v-expansion-panel>
             <v-expansion-panel v-if="selectedDownload.metadata.stderr">
-              <v-expansion-panel-title>Error Log</v-expansion-panel-title>
+              <v-expansion-panel-title>Verbose Log (stderr)</v-expansion-panel-title>
               <v-expansion-panel-text>
-                <pre class="text-caption text-error" style="white-space: pre-wrap; max-height: 300px; overflow: auto">{{ selectedDownload.metadata.stderr }}</pre>
+                <pre class="text-caption" style="white-space: pre-wrap; max-height: 300px; overflow: auto">{{ selectedDownload.metadata.stderr }}</pre>
               </v-expansion-panel-text>
             </v-expansion-panel>
           </v-expansion-panels>