Refactor PostFiles component to use URL parameters and simplify implementation

- Change from hash fragments (#file=2) to URL parameters (?idx=2) for server-side prerendering support
- Simplify React component by removing complex client-side hydration logic
- Remove unnecessary props: totalFiles, hasMultipleFiles (derive from files.length)
- Remove redundant useCallback and popstate handlers
- Update Rails helper to read URL parameter and pass correct initialSelectedIndex
- Maintain all functionality: carousel, keyboard navigation, URL state management

app/helpers/domain/posts_helper.rb

@@ -195,11 +195,12 @@ module Domain::PostsHelper
   end
 
   sig do
-    params(ok_files: T::Array[Domain::PostFile]).returns(
-      T::Hash[Symbol, T.untyped],
-    )
+    params(
+      ok_files: T::Array[Domain::PostFile],
+      initial_file_index: T.nilable(Integer),
+    ).returns(T::Hash[Symbol, T.untyped])
   end
-  def props_for_post_files(ok_files)
+  def props_for_post_files(ok_files, initial_file_index: nil)
     files_data =
       ok_files.map.with_index do |file, index|
         thumbnail_path = nil
@@ -266,12 +267,14 @@ module Domain::PostsHelper
         }
       end
 
-    {
-      files: files_data,
-      totalFiles: ok_files.count,
-      initialSelectedIndex: 0,
-      hasMultipleFiles: ok_files.count > 1,
-    }
+    # Validate initial_file_index
+    validated_initial_index = 0
+    if initial_file_index && initial_file_index >= 0 &&
+         initial_file_index < ok_files.count
+      validated_initial_index = initial_file_index
+    end
+
+    { files: files_data, initialSelectedIndex: validated_initial_index }
   end
 
   sig { params(url: String).returns(T.nilable(String)) }

app/javascript/bundles/Main/components/PostFiles.tsx

@@ -1,5 +1,5 @@
 import * as React from 'react';
-import { useState } from 'react';
+import { useState, useEffect, useCallback } from 'react';
 import { FileCarousel } from './FileCarousel';
 import { DisplayedFile } from './DisplayedFile';
 
@@ -14,31 +14,78 @@ export interface FileData {
 
 interface PostFilesProps {
   files: FileData[];
-  totalFiles: number;
   initialSelectedIndex?: number;
-  hasMultipleFiles: boolean;
 }
 
 export const PostFiles: React.FC<PostFilesProps> = ({
   files,
-  totalFiles,
   initialSelectedIndex = 0,
-  hasMultipleFiles,
 }) => {
   const [selectedIndex, setSelectedIndex] = useState(initialSelectedIndex);
 
+  // Update URL parameter when selected file changes
+  const updateUrlWithFileIndex = (index: number) => {
+    if (typeof window === 'undefined' || files.length <= 1) return;
+
+    const url = new URL(window.location.href);
+    url.searchParams.set('idx', index.toString());
+    window.history.replaceState({}, '', url.toString());
+  };
+
   const handleFileSelect = (fileId: number, index: number) => {
     setSelectedIndex(index);
+    updateUrlWithFileIndex(index);
   };
 
+  const navigateToNextFile = () => {
+    if (files.length > 1) {
+      const nextIndex = (selectedIndex + 1) % files.length;
+      handleFileSelect(files[nextIndex].id, nextIndex);
+    }
+  };
+
+  const navigateToPreviousFile = () => {
+    if (files.length > 1) {
+      const prevIndex = (selectedIndex - 1 + files.length) % files.length;
+      handleFileSelect(files[prevIndex].id, prevIndex);
+    }
+  };
+
+  // Add keyboard navigation
+  useEffect(() => {
+    const handleKeyDown = (event: KeyboardEvent) => {
+      // Only handle arrow keys if we have multiple files
+      if (files.length <= 1) return;
+
+      switch (event.key) {
+        case 'ArrowLeft':
+          event.preventDefault();
+          navigateToPreviousFile();
+          break;
+        case 'ArrowRight':
+          event.preventDefault();
+          navigateToNextFile();
+          break;
+      }
+    };
+
+    // Add event listener to document
+    document.addEventListener('keydown', handleKeyDown);
+
+    // Cleanup event listener on unmount
+    return () => {
+      document.removeEventListener('keydown', handleKeyDown);
+    };
+  }, [selectedIndex, files.length]);
+
   const selectedFile = files[selectedIndex];
 
   return (
     <section id="file-display-section">
-      {hasMultipleFiles && (
+      {files.length > 1 && (
         <FileCarousel
           files={files}
-          totalFiles={totalFiles}
+          totalFiles={files.length}
           selectedIndex={selectedIndex}
           onFileSelect={handleFileSelect}
         />

app/views/domain/posts/default/_section_file.html.erb

@@ -3,11 +3,16 @@
   <% current_file = ok_files.first || post.primary_file_for_view %>
   <% if ok_files.any? && current_file&.log_entry&.status_code == 200 %>
     <!-- React PostFiles Component handles everything -->
+    <%
+      # Extract file index from URL parameter
+      idx_param = params[:idx]
+      initial_file_index = idx_param.present? ? idx_param.to_i : nil
+    %>
     <%= react_component(
           "PostFiles",
           {
             prerender: true,
-            props: props_for_post_files(ok_files),
+            props: props_for_post_files(ok_files, initial_file_index: initial_file_index),
             html_options: {
               id: "post-files-component"
             }