chore: bump prettier-plugin-svelte 3.5.2 -> 4.0.0 (#234)

The 4.0.0 plugin crashes during `prettier --check` when an `{@const}`
contains a ternary whose expressions involve a `BinaryExpression`
(`Error: unknown node type: BinaryExpression`). Move the `totalProgress`
computation out of the `{@const}` in SendButton.svelte into a script-level
`$derived` so prettier can format the file again, then take the major bump.

Breaking-change audit for 4.0.0:
- Requires Svelte 5 (have ^5.55.9).
- Drops `svelteBracketNewLine` and `svelteStrictMode` options — repo uses
  neither in `package.json#prettier`.

Verified locally: `npm run lint`, `svelte-check`, `npm run build` clean.

Author: rubenhensen

package-lock.json

@@ -32,7 +32,7 @@
         "lint-staged": "^17.0.5",
         "mdsvex": "^0.12.7",
         "prettier": "^3.8.3",
-        "prettier-plugin-svelte": "^3.5.2",
+        "prettier-plugin-svelte": "^4.0.0",
         "rollup": "^4.60.4",
         "sass": "^1.99.0",
         "svelte": "^5.55.9",

package.json

@@ -308,6 +308,15 @@
     let buttonRef: HTMLButtonElement | null = $state(null)
     let dialogRef: HTMLDialogElement | null = $state(null)
 
+    let totalProgress = $derived(
+        encryptState.percentages.length > 0
+            ? Math.round(
+                  encryptState.percentages.reduce((a, b) => a + b, 0) /
+                      encryptState.percentages.length
+              )
+            : 0
+    )
+
     $effect(() => {
         if (!browser || !dialogRef) return
         if (showValidationModal) {
@@ -337,13 +346,6 @@
     {/if}
     {#if encryptState.encryptionState === EncryptionState.Encrypting}
         <!-- Loading info box during upload -->
-        {@const totalProgress =
-            encryptState.percentages.length > 0
-                ? Math.round(
-                      encryptState.percentages.reduce((a, b) => a + b, 0) /
-                          encryptState.percentages.length
-                  )
-                : 0}
         <div class="upload-info-box">
             <div
                 class="progress-bar"