chore: declare node>=20 engines floor and document prettier-plugin-svelte workaround

prettier-plugin-svelte@4.0.0 raised its own engines.node to >=20, giving
the repo an undocumented Node 20 floor. Add a top-level engines field so a
contributor on Node 18 gets a clear EBADENGINE signal.

Also document why totalProgress in SendButton.svelte is a script-level
$derived rather than an inline {@const} ternary, linking the upstream bug
(sveltejs/prettier-plugin-svelte#528) so the workaround can be retired.

Closes #235

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: dobby-coder[bot]

package.json

@@ -2,6 +2,9 @@
     "name": "postguard-website",
     "version": "1.7.0",
     "private": true,
+    "engines": {
+        "node": ">=20"
+    },
     "scripts": {
         "dev": "vite dev",
         "build": "vite build",

src/lib/components/filesharing/SendButton.svelte

@@ -304,6 +304,11 @@
     let buttonRef: HTMLButtonElement | null = $state(null)
     let dialogRef: HTMLDialogElement | null = $state(null)
 
+    // Computed here as a script-level $derived rather than inline in the template
+    // via {@const} to dodge an upstream prettier-plugin-svelte crash on ternaries
+    // with a BinaryExpression test inside {@const}
+    // (https://github.com/sveltejs/prettier-plugin-svelte/issues/528). Once that
+    // fix is confirmed in the pinned plugin version, this can move back inline.
     let totalProgress = $derived(
         encryptState.percentages.length > 0
             ? Math.round(