feat(subs-hub): collapse /credentials + /downloads into /subscriptions hub with three GS-style subtabs

Replaces the three top-level routes with a single `/subscriptions` parent
owning the whole download-pipeline domain. Internal tab state via `?tab=`
query param, mirroring ArtistView's pattern. TopNav auto-drops the two
removed entries (route-driven via meta.title). Bookmark-safe redirects
from `/credentials` and `/downloads` route into the appropriate subtab.

**Subtab 1 — Subscriptions (default).** Carries over the existing
artist-grouped expandable table; adds (a) status filter dropdown, (b)
bulk-select column with Enable/Disable/Delete-all actions, (c) GS-style
color-coded `PlatformChip` per distinct platform in the collapsed row.
Reuses SourceRow, SourceHealthDot, SourceFormDialog, ArtistCreateDialog.

**Subtab 2 — Downloads.** Full GS dashboard. Five colored stat chips up
top (Queued/Running/Completed/Failed/Skipped, sourced from new
`GET /api/downloads/stats?window_hours=`). Popover-style filter UI
(Status/Source/FromDate/ToDate) with active-filter pills below.
Maintenance menu wraps existing /api/import/retry-failed and
/api/import/clear-stuck endpoints; Export-failed-logs item disabled with
a "v2" tooltip. Per-row Retry preserved via existing DownloadEventRow.

**Subtab 3 — Settings.** Four sections: ExtensionKeyBar (top), GS-style
per-platform CredentialCard grid (md=6 v-row/v-col, dashed border if
unset / accent border if set, expandable how-to panel), Downloader card
(rate limit, validate_files), Schedule defaults card (default interval,
event retention, failure warning threshold). The Downloader and Schedule
sections were extracted out of components/settings/ImportFiltersForm.vue
— SettingsView's Import tab now owns only image-import filters.

**Backend:** new `GET /api/downloads/stats` returns
{pending, running, ok, error, skipped} count grouped by status over the
configurable window. Status keys stay raw from the ENUM; UI does the
display-label mapping. Two integration tests pin the response shape +
window_hours validation.

**Util:** `frontend/src/utils/platformColor.js` — single source of truth
for the six platforms' color + icon + label, mirroring GS's palette
(patreon=red mdi-patreon, subscribestar=amber mdi-star,
hentaifoundry=purple mdi-palette, discord=indigo mdi-discord,
pixiv=blue mdi-alpha-p-box, deviantart=green mdi-deviantart). Unknown
platform falls back to grey + mdi-web.

Deferred (explicit non-goals): subscription import/export, "Trigger Due
Now" scheduler-tick button (needs new backend endpoint), Export Failed
Logs CSV dump.

frontend/src/stores/downloads.js

@@ -8,10 +8,14 @@ export const useDownloadsStore = defineStore('downloads', () => {
   const events = ref([])
   const cursor = ref(null)
   const hasMore = ref(true)
-  const filter = ref({ status: null, source_id: null, artist_id: null })
+  const filter = ref({
+    status: null, source_id: null, artist_id: null,
+    from_date: null, to_date: null,
+  })
   const selected = ref(null)
   const loading = ref(false)
   const error = ref(null)
+  const stats = ref({ pending: 0, running: 0, ok: 0, error: 0, skipped: 0 })
 
   function _params(extra = {}) {
     const out = { limit: 50, ...extra }
@@ -65,8 +69,13 @@ export const useDownloadsStore = defineStore('downloads', () => {
     selected.value = null
   }
 
+  async function loadStats(windowHours = 24) {
+    stats.value = await api.get('/api/downloads/stats', { params: { window_hours: windowHours } })
+    return stats.value
+  }
+
   return {
-    events, cursor, hasMore, filter, selected, loading, error,
-    loadFirst, loadMore, loadOne, applyFilter, closeDetail,
+    events, cursor, hasMore, filter, selected, loading, error, stats,
+    loadFirst, loadMore, loadOne, applyFilter, closeDetail, loadStats,
   }
 })