fix(frontend): warn about FAT32, not exFAT, for an installer over 4 GiB

SPEC.md

@@ -560,7 +560,7 @@ All of them warn, and none of them blocks the button:
 
 - there is not a single `.exe` in the archive (the select is empty, there is nothing to build)
 - the UTF-8 bit 11 is not set **and** there are non-ASCII names → Cyrillic names will arrive garbled, suggest 7-Zip
-- the size is > 4 GB → a reminder about exFAT
+- the size is 4 GiB or more → the installer does not fit on FAT32, which caps a file a byte short of 4 GiB and is how USB sticks and memory cards up to 32 GB usually come; exFAT has no such limit
 - `navigator.storage.estimate()` < the size of the archive → there will not be enough space (Chrome writes into a temporary swap file and swaps it in at the end). Only where the picker is: Firefox 157 reports the quota of the site instead, 7.4 GiB on a disk with 51 GB free
 - the chosen exe got a negative weight under 7.2.1 (the person picked by hand what we ranked down): warn, but do not stand in the way
 - the artwork is not filled in

packages/frontend/app/services/checks/checks.spec.ts

@@ -40,13 +40,23 @@ describe('services', () => {
         expect(messages.join(' ')).toContain('7-Zip')
       })
 
-      it('should mention exFAT past 4 GiB', () => {
+      it('should warn about FAT32 once the archive reaches 4 GiB, and point to exFAT', () => {
         const messages = preflight({
           ...CLEAN,
-          archiveSize: 5 * 1024 ** 3
+          archiveSize: 4 * 1024 ** 3
         }).map(check => check.message)
 
-        expect(messages.join(' ')).toContain('exFAT')
+        expect(messages.join(' ')).toContain('too big for a drive formatted as FAT32')
+        expect(messages.join(' ')).toContain('Use one formatted as exFAT')
+      })
+
+      it('should leave an archive of 4 GB in decimal units alone: FAT32 holds it', () => {
+        const messages = preflight({
+          ...CLEAN,
+          archiveSize: 4_000_000_000
+        }).map(check => check.message)
+
+        expect(messages.join(' ')).not.toContain('FAT32')
       })
 
       it('should warn when the browser has less free space than the archive', () => {

packages/frontend/app/services/checks/checks.ts

@@ -4,8 +4,13 @@ import type {
   CheckInput
 } from './checks.types.ts'
 
-/** 4 GiB: a single file past this does not fit on a default-formatted exFAT card. */
-const EXFAT_FILE_LIMIT = 4294967296
+/**
+ * The largest file FAT32 holds, a byte short of 4 GiB. exFAT has no such limit,
+ * but USB sticks and memory cards up to 32 GB usually come formatted as FAT32.
+ * The check goes by the archive, which the installer outgrows only by its
+ * header and meta: an archive that close to the limit is the one case it misses.
+ */
+const FAT32_FILE_LIMIT = 4294967295
 
 /**
  * Everything here warns and nothing blocks (6.3). Games are strange, and a
@@ -32,11 +37,12 @@ export function preflight(input: CheckInput): Check[] {
     })
   }
 
-  if (input.archiveSize > EXFAT_FILE_LIMIT) {
+  if (input.archiveSize > FAT32_FILE_LIMIT) {
     checks.push({
       level: 'note',
-      message: `The archive is ${formatSize(input.archiveSize)}. A single file over 4 GB does not fit `
-        + 'on an exFAT card formatted with the default settings - check the card before copying.'
+      message: `The archive is ${formatSize(input.archiveSize)}, so the installer will be a little bigger: `
+        + 'too big for a drive formatted as FAT32, as USB sticks and memory cards up to 32 GB usually are. '
+        + 'Use one formatted as exFAT, or copy the file over the network.'
     })
   }