Add Finished Books feature with episode playlists and nav dropdown

Public:
- /finished page with ornament heading, intro text, book grid, and styled empty state
- /finished/:id episode playlist page filtered by RSS season number
- Episodes nav becomes click-toggle dropdown (Current Series / Finished Books)
- Finished Books link in footer
- Dropdown font fixed to match other nav links

Admin:
- End Current Series & Start New Book wizard on Current Series tab
- Finished Books management tab with add/edit/remove and image asset picker

Data:
- FinishedBook type + finishedBooks[] field on SiteContent
- RSS parser extracts itunes:season per episode

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/App.css

@@ -190,7 +190,8 @@
   cursor: pointer;
 }
 
-.header-nav a {
+.header-nav a,
+.header-nav button {
   color: var(--brand-muted);
   text-decoration: none;
   font-family: var(--brand-font-body);
@@ -8701,18 +8702,29 @@
   display: flex;
   align-items: center;
   gap: 0.25rem;
+  color: var(--brand-muted);
+  font-family: inherit;
+  font-weight: 500;
+  letter-spacing: 0.06em;
+  text-transform: uppercase;
+  font-size: 0.96rem;
+  line-height: inherit;
+  transition: color 200ms;
+}
+
+.nav-dropdown-trigger:hover,
+.nav-dropdown--open .nav-dropdown-trigger {
+  color: var(--brand-gold);
 }
 
 .nav-dropdown-arrow {
   font-size: 0.7em;
   opacity: 0.7;
-  transition: transform 200ms;
 }
 
 .nav-dropdown-menu {
-  display: none;
   position: absolute;
-  top: calc(100% + 0.5rem);
+  top: calc(100% + 0.75rem);
   left: 50%;
   transform: translateX(-50%);
   background: rgba(15, 15, 12, 0.98);
@@ -8724,11 +8736,6 @@
   box-shadow: 0 8px 24px rgba(0,0,0,0.5);
 }
 
-.nav-dropdown:hover .nav-dropdown-menu,
-.nav-dropdown:focus-within .nav-dropdown-menu {
-  display: block;
-}
-
 .nav-dropdown-item {
   display: block;
   padding: 0.55rem 1.1rem;
@@ -8993,3 +9000,52 @@
     height: 120px;
   }
 }
+
+/* ── Finished Books page section ── */
+.section-finished-books {
+  min-height: 60vh;
+  padding: 4rem 0 6rem;
+}
+
+.finished-books-header {
+  text-align: center;
+  margin-bottom: 3rem;
+}
+
+.finished-books-intro {
+  color: var(--brand-muted);
+  font-size: 1.05rem;
+  max-width: 540px;
+  margin: -1rem auto 0;
+  line-height: 1.65;
+}
+
+.finished-books-empty-state {
+  text-align: center;
+  padding: 4rem 2rem 5rem;
+  border: 1px solid rgba(201, 168, 76, 0.15);
+  border-radius: 16px;
+  background: rgba(26, 26, 21, 0.5);
+  max-width: 520px;
+  margin: 0 auto;
+}
+
+.finished-books-empty-icon {
+  font-size: 3rem;
+  margin-bottom: 1.25rem;
+  opacity: 0.6;
+}
+
+.finished-books-empty-heading {
+  font-family: var(--brand-font-heading);
+  font-size: 1.6rem;
+  color: var(--brand-warm-white);
+  margin: 0 0 0.75rem;
+}
+
+.finished-books-empty-text {
+  color: var(--brand-muted);
+  font-size: 0.97rem;
+  line-height: 1.65;
+  margin: 0;
+}