Derive the vault key from the mnemonic's words however they are spaced (closes #135)

The long-term key was derived from the mnemonic text exactly as typed,
so a double space, a tab or a trailing newline gave a different key,
and a vault made that way did not open with the same words typed
normally. agehd.DeriveEntropy, which init, vault create, vault import,
unlocker add and every unlock go through, now joins the words with
single spaces and puts them in Unicode NFKD, as BIP39 defines the
seed's input, before hashing them. golang.org/x/text, already an
indirect dependency, becomes a direct one for the NFKD step.

Judgement call: internal/bip39's NewSeed keeps hashing its input as
given, as upstream does; the normalising is in agehd.

Model: opus-5-5

TODO.md

@@ -18,6 +18,13 @@ https://git.eeqj.de/sneak/secret/milestone/12
 
 # Completed Steps
 
+- 2026-10-12: A mnemonic typed with extra spaces, tabs or line breaks gives the
+  same long-term key as its words separated by single spaces
+  (https://git.eeqj.de/sneak/secret/issues/135). The key was derived from the
+  text exactly as typed, so a vault made with a double or trailing space did not
+  open with the same words typed normally. `agehd.DeriveEntropy`, which every
+  use of the mnemonic goes through, now joins the words with single spaces and
+  puts them in Unicode NFKD, as BIP39 defines, before making the seed.
 - 2026-10-12: The keychain and Secure Enclave unlockers are gone from the code
   and the documents (https://git.eeqj.de/sneak/secret/issues/132). They were
   research: neither works without macOS code signing and entitlements, which