fix(bootstrap): report verify progress correctly past 2 GiB

The checksum and MD5 passes measured the file for their progress bar
with fseek/ftell. On Windows a long is 32-bit, so ftell overflows on
any archive larger than 2 GiB and hands back a wrong (often negative)
size, which makes the verify progress percentage meaningless on exactly
the multi-gigabyte downloads this code exists for.

- Both passes now take the size from std::filesystem::file_size, which
  is 64-bit on every platform, through a small helper that returns 0 if
  the size can't be read.
- No seek is performed just to size the file.

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

src/util/bootstrap.cpp

@@ -116,6 +116,13 @@ static std::uint64_t satAdd(std::uint64_t a, std::uint64_t b) {
     return (a > UINT64_MAX - b) ? UINT64_MAX : a + b;
 }
 
+// fs::file_size is 64-bit on every platform; ftell() returns a 32-bit long on Win LLP64.
+static long long fileSizeOrZero(const std::string& path) {
+    std::error_code ec;
+    auto sz = fs::file_size(path, ec);
+    return ec ? 0 : (long long)sz;
+}
+
 static std::string lowerAscii(std::string s) {
     for (char& c : s)
         if (c >= 'A' && c <= 'Z') c = static_cast<char>(c - 'A' + 'a');
@@ -924,10 +931,8 @@ std::string Bootstrap::computeSHA256(const std::string& filePath, float pctBase,
     FILE* fp = fopen(filePath.c_str(), "rb");
     if (!fp) return {};
 
-    // Get file size for progress reporting
-    fseek(fp, 0, SEEK_END);
-    long long fileSize = ftell(fp);
-    fseek(fp, 0, SEEK_SET);
+    // File size for progress reporting (not a seek: ftell is 32-bit on Windows LLP64).
+    const long long fileSize = fileSizeOrZero(filePath);
 
     crypto_hash_sha256_state state;
     crypto_hash_sha256_init(&state);
@@ -1061,10 +1066,8 @@ std::string Bootstrap::computeMD5(const std::string& filePath, float pctBase, fl
     FILE* fp = fopen(filePath.c_str(), "rb");
     if (!fp) return {};
 
-    // Get file size for progress reporting
-    fseek(fp, 0, SEEK_END);
-    long long fileSize = ftell(fp);
-    fseek(fp, 0, SEEK_SET);
+    // File size for progress reporting (not a seek: ftell is 32-bit on Windows LLP64).
+    const long long fileSize = fileSizeOrZero(filePath);
 
     MD5Context ctx;
     md5_init(&ctx);