vault: files sealed under a password that is never stored

Argon2id turns the vault password into a key that seals a random master
key; only the sealed copy is kept. Each file gets its own key, sealed
under the master key in its header with its media id, and its bytes are
AES-256-GCM STREAM records, so a swapped, cut or altered file refuses
to open. Uploads with ?encrypt=true are sealed as they stream in.

:vault, :unlock, :lock, :passwd, :encrypt, :decrypt and :upload! drive
it. The vault locks after WORKSPACE_VAULT_IDLE_MIN idle minutes, on
logout and on restart; while locked, sealed files answer 423, share
links never serve them, and :export! refuses rather than leave them out.

Sealing an existing file rewrites it under a new id and retires the old
one: the mirror deletes the plaintext from Proton Drive outright,
trash included, whatever WORKSPACE_BACKUP_PRUNE says, and drops it from
the readable files/ view. The mirror now compares on-disk sizes, so
sealed files are not re-hashed every run.

web/src/lib/StatusBar.svelte

@@ -29,6 +29,7 @@
   <span class="msg" class:alarm={workspace.flash?.kind === 'error'}>{workspace.flash?.text ?? ''}</span>
 
   {#if workspace.fullscreen}<span class="flag">full</span>{/if}
+  {#if workspace.vault?.unlocked}<span class="flag vault" title="the vault is open — :lock closes it">vault open</span>{/if}
   {#if lines !== null}<span class="readout">{lines}<span class="unit">L</span></span>{/if}
   <span class="hint">
     {isTouchDevice ? 'tap : for commands' : '? help · : command · / search'}