Add a hash-based password bypass link

The upload password gate can now be skipped with a bookmarkable
#psst=<sha256-hash> fragment instead of typing the password each time.
The server accepts either the plaintext password (X-Upload-Password) or
its SHA-256 hash (X-Upload-Password-Hash), compared timing-safely. The
client computes the hash locally after a normal password entry, so the
bypass link never contains the real password — only a one-way digest of
it, kept out of the URL query string (and thus server logs) by living in
the fragment, same as the decryption key.

client/src/api.ts

@@ -1,7 +1,15 @@
-export async function checkPassword(password: string): Promise<boolean> {
+export type Credential = { kind: 'password' | 'hash'; value: string }
+
+function credentialHeaders(cred: Credential): Record<string, string> {
+  return cred.kind === 'password'
+    ? { 'X-Upload-Password': cred.value }
+    : { 'X-Upload-Password-Hash': cred.value }
+}
+
+export async function checkCredential(cred: Credential): Promise<boolean> {
   const res = await fetch('/api/auth/check', {
     method: 'POST',
-    headers: { 'X-Upload-Password': password },
+    headers: credentialHeaders(cred),
   })
   const body = (await res.json()) as { ok: boolean }
   return body.ok
@@ -9,13 +17,13 @@ export async function checkPassword(password: string): Promise<boolean> {
 
 export async function uploadDrop(
   ciphertext: Uint8Array,
-  password: string,
+  cred: Credential,
 ): Promise<{ id: string; ttl: number }> {
   const res = await fetch('/api/drops', {
     method: 'POST',
     headers: {
       'Content-Type': 'application/octet-stream',
-      'X-Upload-Password': password,
+      ...credentialHeaders(cred),
     },
     body: ciphertext.slice().buffer,
   })