refactor(frontend): add getHeaderValue wire-shape lookup to lib/xray/headers

Tiny piece of the toShareLink scaffold. The legacy Inbound.getHeader(obj,
name) iterated the panel's internal HeaderEntry[] form; the new
getHeaderValue reads the Record<string, string|string[]> map our Zod
schemas store on the wire. Case-insensitive, returns '' on miss to match
the legacy fallback so link-generator call sites stay simple.

For repeated-name maps (TCP/WS-style string[] values) the first value
wins — matches the legacy iteration order so the share URL's Host hint
stays deterministic.

Five unit tests cover undefined/null/empty inputs, case folding,
string-valued and array-valued matches, empty-array edge case, and
missing-key fallback. Suite: 64 tests across 6 files; typecheck + lint
clean.

This unblocks the next slice: per-protocol link generators (genVmessLink
etc.) take a typed inbound + client and call getHeaderValue against the
ws/httpupgrade/xhttp/tcp.request header maps.

frontend/src/lib/xray/headers.ts

@@ -32,6 +32,27 @@ export function toHeaders(v2Headers: unknown): HeaderEntry[] {
   return out;
 }
 
+// Case-insensitive lookup against a wire-shape header map. The legacy
+// `Inbound.getHeader(obj, name)` iterated `obj.headers` as a HeaderEntry[];
+// this version reads the Record map our Zod schemas store. For repeated
+// header names (string[] in TCP/WS-style maps) the first value wins —
+// matches the legacy iteration order. Returns '' when missing, mirroring
+// the legacy fallback so link-generator call sites stay simple.
+export function getHeaderValue(
+  headers: Readonly<Record<string, string | string[]>> | undefined | null,
+  name: string,
+): string {
+  if (!headers || typeof headers !== 'object') return '';
+  const lower = name.toLowerCase();
+  for (const key of Object.keys(headers)) {
+    if (key.toLowerCase() !== lower) continue;
+    const value = headers[key];
+    if (typeof value === 'string') return value;
+    if (Array.isArray(value)) return value[0] ?? '';
+  }
+  return '';
+}
+
 // Collapse a HeaderEntry[] back into a V2-style header map. When `arr` is
 // true (the default — matches Xray's TCP/WS/HTTP request/response shape),
 // duplicate header names accumulate into a string[]. When false (used for

frontend/src/test/headers.test.ts

@@ -1,6 +1,6 @@
 import { describe, expect, it } from 'vitest';
 
-import { toHeaders, toV2Headers, type HeaderEntry } from '@/lib/xray/headers';
+import { getHeaderValue, toHeaders, toV2Headers, type HeaderEntry } from '@/lib/xray/headers';
 import { XrayCommonClass } from '@/models/inbound';
 
 // Shadow harness: the new pure helpers must agree byte-for-byte with the
@@ -57,3 +57,28 @@ describe('toV2Headers parity (arr=false)', () => {
     });
   }
 });
+
+describe('getHeaderValue lookups', () => {
+  it('returns empty string for missing map', () => {
+    expect(getHeaderValue(undefined, 'host')).toBe('');
+    expect(getHeaderValue(null, 'host')).toBe('');
+    expect(getHeaderValue({}, 'host')).toBe('');
+  });
+
+  it('finds a string-valued header case-insensitively', () => {
+    expect(getHeaderValue({ Host: 'example.test' }, 'host')).toBe('example.test');
+    expect(getHeaderValue({ host: 'example.test' }, 'HOST')).toBe('example.test');
+  });
+
+  it('returns first value when the header is an array', () => {
+    expect(getHeaderValue({ Accept: ['text/html', 'application/json'] }, 'accept')).toBe('text/html');
+  });
+
+  it('returns empty string when the header has empty array', () => {
+    expect(getHeaderValue({ Host: [] }, 'host')).toBe('');
+  });
+
+  it('returns empty string for missing header name', () => {
+    expect(getHeaderValue({ Host: 'x' }, 'origin')).toBe('');
+  });
+});