feat(ui): admin-only file location and metadata editing on book detail

The book detail page exposed server internals and unusable controls to
everyday users: it now computes and renders the book's absolute on-disk
location, and gates all of it behind the admin role.

- The page handler resolves library folder + relative path (verified
  with os.Stat; falls back to the relative path when the file is not
  found on disk) into the new MediaDetail.FileLocation field - only for
  admins, so the absolute path never leaves the server for regular
  users. This also makes it possible to locate sparse entries whose
  metadata rows are largely blank.
- The Metadata grid gains a full-width, monospace, click-selectable
  Location row (admins only).
- The Edit button and the MetadataEditorModal markup render only for
  admins. The modal drives admin-only endpoints (metadata PUT, rescan,
  reset), so non-admins previously saw a button and a form that could
  only ever fail with 403s.

templates/book_detail.templ

@@ -107,10 +107,12 @@ templ BookDetail(user User, book handlers.MediaDetail, errorMessage string) {
 										<span class="badge ml-1" style="background-color: var(--accent-muted); color: var(--accent);">{ book.NotesCount + book.HighlightsCount }</span>
 									}
 								</button>
+							if user.Role == "admin" {
 								<button @click="showMetadataEditor()" class="btn btn-secondary px-5 py-2.5">
 									@Icon("edit", "h-4 w-4")
 									<span>Edit</span>
 								</button>
+							}
 							</div>
 							<!-- Rating -->
 							<div class="mb-5" @mouseleave="ratingHover = 0">
@@ -370,12 +372,18 @@ templ BookDetail(user User, book handlers.MediaDetail, errorMessage string) {
 							<p class="text-xs uppercase tracking-wide" style="color: var(--text-secondary)">Format</p>
 							<p style="color: var(--text-primary)">{ book.MimeType.String }</p>
 						</div>
-						if book.FileSize.Valid && book.FileSize.Int64 > 0 {
-							<div>
-								<p class="text-xs uppercase tracking-wide" style="color: var(--text-secondary)">File Size</p>
-								<p style="color: var(--text-primary)">{ formatFileSize(book.FileSize.Int64) }</p>
-							</div>
-						}
+					if book.FileSize.Valid && book.FileSize.Int64 > 0 {
+						<div>
+							<p class="text-xs uppercase tracking-wide" style="color: var(--text-secondary)">File Size</p>
+							<p style="color: var(--text-primary)">{ formatFileSize(book.FileSize.Int64) }</p>
+						</div>
+					}
+					if user.Role == "admin" {
+						<div class="md:col-span-2 lg:col-span-3">
+							<p class="text-xs uppercase tracking-wide" style="color: var(--text-secondary)">Location</p>
+							<p class="font-mono text-xs break-all select-all" style="color: var(--text-secondary)">{ book.FileLocation }</p>
+						</div>
+					}
 					</div>
 					if book.GoodreadsID.Valid || book.OpenlibraryID.Valid || book.GoogleBooksID.Valid || book.Asin.Valid || book.Isbn.Valid || book.WebUrl.Valid {
 						<div class="mt-5 pt-4 border-t flex flex-wrap gap-2" style="border-color: color-mix(in srgb, var(--text-primary) 7%, transparent);">
@@ -426,7 +434,9 @@ templ BookDetail(user User, book handlers.MediaDetail, errorMessage string) {
 			</div>
 			@ProgressSyncModal(user, book)
 			@NotesHighlightsModal(user, book)
-			@MetadataEditorModal(book)
+			if user.Role == "admin" {
+				@MetadataEditorModal(book)
+			}
 			@ErrorToast(errorMessage)
 		</body>
 	</html>