build(docker): upgrade pgvector image to pg18

Upgrades all references of pgvector/pgvector from pg16 to pg18,
including docker-compose files and e2e testing environments.
Updates the development docker-compose to mount the postgres volume
at /var/lib/postgresql to align with pg18+ directory requirements.

Also adds `shrink-0` to WhatsNewDialog header and footer to resolve
a layout overflow glitch.

client/src/features/library/components/BookUploadModal.vue

@@ -7,7 +7,7 @@ import { useAppInfo } from '@/features/settings/composables/useAppInfo'
 import { SUPPORTED_FORMATS, SUPPORTED_FORMATS_ACCEPT, useBookUpload, type FileUploadStatus } from '../composables/useBookUpload'
 import { emitLibraryUploadCompleted } from '../composables/useLibraryUploadEvents'
 import { useLibraries } from '../composables/useLibraries'
-
+import { api } from '@/lib/api'
 const props = defineProps<{
   libraryId?: number
 }>()

client/src/features/whats-new/WhatsNewDialog.vue

@@ -53,7 +53,7 @@ useModal({
         ref="panel"
         class="flex max-h-[90vh] w-full max-w-2xl flex-col rounded-t-2xl border border-border bg-card shadow-xl motion-safe:animate-in motion-safe:slide-in-from-bottom-4 sm:rounded-2xl"
       >
-        <header class="relative overflow-hidden border-b border-border px-5 pb-4 pt-5">
+        <header class="relative shrink-0 overflow-hidden border-b border-border px-5 pb-4 pt-5">
           <div
             class="pointer-events-none absolute inset-0"
             style="
@@ -134,7 +134,7 @@ useModal({
           </RouterLink>
         </div>
 
-        <footer class="flex items-center justify-end gap-3 border-t border-border px-5 py-4">
+        <footer class="flex shrink-0 items-center justify-end gap-3 border-t border-border px-5 py-4">
           <button
             type="button"
             class="rounded-md border border-border px-4 py-2 text-sm font-medium text-foreground transition-colors hover:bg-muted"