refactor: defer recipe image upload until form submission

Changed recipe image upload behavior to only process images when the
form is submitted, rather than immediately on file selection. This
prevents orphaned image files when users abandon the form.

Changes:
- CardAdd.svelte: Preview only, store File object instead of uploading
- Created imageProcessing.ts: Shared utility for image processing
- Add/edit page clients: Use selected_image_file instead of filename
- Add/edit page servers: Process and save images during form submission
- Images are validated, hashed, and saved in multiple formats on submit

Benefits:
- No orphaned files from abandoned forms
- Faster initial file selection experience
- Server-side image processing ensures security validation
- Cleaner architecture with shared processing logic

src/routes/[recipeLang=recipeLang]/edit/[name]/+page.svelte

@@ -27,7 +27,7 @@
 		"https://bocken.org/static/rezepte/thumb/" +
 			(data.recipe.images?.[0]?.mediapath || `${data.recipe.short_name}.webp`)
 	);
-	let uploaded_image_filename = $state("");
+	let selected_image_file = $state<File | null>(null);
 
 	// Translation workflow state
 	let showTranslationWorkflow = $state(false);
@@ -109,10 +109,10 @@
 	let currentRecipeData = $derived.by(() => {
 		// Ensure we always have a valid images array with at least one item
 		let recipeImages;
-		if (uploaded_image_filename) {
-			// New image uploaded
+		if (selected_image_file) {
+			// New image selected (will be uploaded on form submission)
 			recipeImages = [{
-				mediapath: uploaded_image_filename,
+				mediapath: 'pending',
 				alt: images[0]?.alt || "",
 				caption: images[0]?.caption || ""
 			}];
@@ -371,9 +371,14 @@
 <form
 	method="POST"
 	bind:this={formElement}
+	enctype="multipart/form-data"
 	use:enhance={() => {
 		submitting = true;
-		return async ({ update }) => {
+		return async ({ update, formData }) => {
+			// Append the image file if one was selected
+			if (selected_image_file) {
+				formData.append('recipe_image', selected_image_file);
+			}
 			await update();
 			submitting = false;
 		};
@@ -381,7 +386,7 @@
 >
 	<!-- Hidden inputs for tracking -->
 	<input type="hidden" name="original_short_name" value={old_short_name} />
-	<input type="hidden" name="keep_existing_image" value={uploaded_image_filename ? "false" : "true"} />
+	<input type="hidden" name="keep_existing_image" value={selected_image_file ? "false" : "true"} />
 	<input type="hidden" name="existing_image_path" value={images[0]?.mediapath || `${old_short_name}.webp`} />
 
 	<!-- Hidden inputs for complex nested data -->
@@ -390,7 +395,6 @@
 	<input type="hidden" name="add_info_json" value={JSON.stringify(add_info)} />
 	<input type="hidden" name="season" value={JSON.stringify(season_local)} />
 	<input type="hidden" name="tags" value={JSON.stringify(card_data.tags)} />
-	<input type="hidden" name="uploaded_image_filename" value={uploaded_image_filename} />
 	<input type="hidden" name="datecreated" value={datecreated?.toString()} />
 
 	<!-- Translation data (updated after approval or marked needs_update) -->
@@ -405,7 +409,7 @@
 	<CardAdd
 		bind:card_data
 		bind:image_preview_url
-		bind:uploaded_image_filename
+		bind:selected_image_file
 		short_name={short_name}
 	/>