perf: share links use scrypt 2^15, about 40 ms each way

Keys keeping to share.keyRules are random and long, so age's 2^18
(a third of a second to share and to open) bought little.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

internal/ui/app/sharefiles_test.go

@@ -17,8 +17,8 @@ import (
 	"git.kebcloud.no/keb/lazysecrets/internal/share"
 )
 
-// scryptWait is how long the driver waits for encrypting and decrypting,
-// which take about a second on purpose.
+// scryptWait is how long the driver waits for encrypting and decrypting:
+// tens of milliseconds, more than its usual wait, and more under load.
 const scryptWait = 20 * time.Second
 
 // demoRules take the demo key, made up to be remembered.