Fix load filter dropdown positioning on bookshelf page

The load filter dropdown was being cut off when the button was positioned
on the left side of the screen due to static right-0 alignment. This became
more problematic as the button position changes with window resize.

Changes:
- Added dynamic dropdown alignment calculation based on button position
  and available viewport space
- Implemented smart positioning logic that checks available space on both
  left and right sides before deciding alignment
- Added window resize listener using requestAnimationFrame to dynamically
  update dropdown position while open
- Added data-load-filter-btn attribute for reliable DOM querying
- Changed from static right-0 to dynamic :class binding for left/right
  alignment

Technical details:
- Alpine.js state: dropdownAlign tracks current alignment (left/right)
- calculateAlignment() method computes button position and available space
- Uses getBoundingClientRect() to measure button position relative to viewport
- Prefers side with >=320px space, otherwise chooses larger side
- requestAnimationFrame ensures smooth updates during resize without
  performance degradation

Fixes issue where dropdown extends beyond viewport edge when button
is near left or right edge of screen.

templates/bookshelf.templ

@@ -247,6 +247,7 @@ templ BookShelf(
 							<div class="flex items-end relative">
 								<button
 									@click="toggleFiltersDropdown()"
+									data-load-filter-btn
 									class="px-4 py-2 rounded-lg font-medium border"
 									style="border-color: var(--border); color: var(--text-primary);"
 								>
@@ -262,7 +263,8 @@ templ BookShelf(
 									x-transition:leave="transition ease-in duration-150"
 									x-transition:leave-start="opacity-100 scale-100"
 									x-transition:leave-end="opacity-0 scale-95"
-									class="absolute top-full mt-2 right-0 w-80 rounded-lg shadow-lg z-50"
+									class="absolute top-full mt-2 w-80 rounded-lg shadow-lg z-50"
+									:class="dropdownAlign === 'left' ? 'left-0' : 'right-0'"
 									style="background-color: var(--bg-secondary); border: 1px solid var(--border); display: none;"
 								>
 									<div class="p-4">