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]/add/+page.svelte

@@ -18,7 +18,7 @@
 	let preamble = $state("");
 	let addendum = $state("");
 	let image_preview_url = $state("");
-	let uploaded_image_filename = $state("");
+	let selected_image_file = $state<File | null>(null);
 
 	// Translation workflow state
 	let showTranslationWorkflow = $state(false);
@@ -91,7 +91,7 @@
 	let germanRecipeData = $derived({
 		...card_data,
 		...add_info,
-		images: uploaded_image_filename ? [{ mediapath: uploaded_image_filename, alt: "", caption: "" }] : [],
+		images: selected_image_file ? [{ mediapath: 'pending', alt: "", caption: "" }] : [],
 		season: season_local,
 		short_name: short_name.trim(),
 		portions: portions_local,
@@ -291,9 +291,14 @@ button.action_button {
 <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;
 		};
@@ -305,7 +310,6 @@ button.action_button {
 	<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} />
 
 	<!-- Translation data (added after approval) -->
 	{#if translationData}
@@ -319,7 +323,7 @@ button.action_button {
 	<CardAdd
 		bind:card_data
 		bind:image_preview_url
-		bind:uploaded_image_filename
+		bind:selected_image_file
 		short_name={short_name}
 	/>