refactor: centralize raw HTTP parsing and header serialization

- Add internal/util/rawhttp.go with ParseRawRequest and SortedHeaderLines
- Refactor intercept/format.go and ui/intercept/helpers.go to use them
- Eliminates duplicated bufio.Reader + textproto parsing spread across 3+ files

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

internal/util/rawhttp.go

@@ -0,0 +1,86 @@
+package util
+
+import (
+	"fmt"
+	"net/http"
+	"sort"
+	"strings"
+)
+
+// RawRequest holds a parsed raw HTTP request string.
+type RawRequest struct {
+	Method  string
+	Path    string
+	Proto   string
+	Host    string
+	Headers []RawHeader
+	Body    string
+}
+
+// RawHeader is a single header key/value pair preserving insertion order.
+type RawHeader struct {
+	Key   string
+	Value string
+}
+
+// ParseRawRequest parses a raw HTTP request string (as produced by
+// FormatRawRequest). The Host header, if present, is extracted into Host
+// but also kept in Headers.
+func ParseRawRequest(raw string) RawRequest {
+	lines := strings.Split(strings.ReplaceAll(raw, "\r\n", "\n"), "\n")
+	var r RawRequest
+	if len(lines) == 0 {
+		return r
+	}
+
+	parts := strings.SplitN(lines[0], " ", 3)
+	if len(parts) >= 1 {
+		r.Method = strings.TrimSpace(parts[0])
+	}
+	if len(parts) >= 2 {
+		r.Path = strings.TrimSpace(parts[1])
+	}
+	if len(parts) >= 3 {
+		r.Proto = strings.TrimSpace(parts[2])
+	}
+
+	i := 1
+	for i < len(lines) {
+		line := strings.TrimRight(lines[i], "\r")
+		if line == "" {
+			i++
+			break
+		}
+		if kv := strings.SplitN(line, ": ", 2); len(kv) == 2 {
+			k := strings.TrimSpace(kv[0])
+			v := strings.TrimSpace(kv[1])
+			r.Headers = append(r.Headers, RawHeader{k, v})
+			if strings.EqualFold(k, "host") {
+				r.Host = v
+			}
+		}
+		i++
+	}
+
+	if i < len(lines) {
+		r.Body = strings.TrimRight(strings.Join(lines[i:], "\n"), "\n")
+	}
+	return r
+}
+
+// SortedHeaderLines returns header lines sorted by key name, formatted as
+// "Key: Value\n" strings. Useful for deterministic serialisation.
+func SortedHeaderLines(h http.Header) []string {
+	keys := make([]string, 0, len(h))
+	for k := range h {
+		keys = append(keys, k)
+	}
+	sort.Strings(keys)
+	var out []string
+	for _, k := range keys {
+		for _, v := range h[k] {
+			out = append(out, fmt.Sprintf("%s: %s\n", k, v))
+		}
+	}
+	return out
+}