fix(web): allow clearing the full name in profile settings (#40186)

Clearing "Full Name" in the profile settings shows success but keeps the
old value: since #31959 it is only updated when non-empty, because the
input is disabled (not submitted) when `change_full_name` is disabled.

Use `*string` for the field, so a missing field keeps the value and an
empty one clears it. With `change_full_name` disabled, any submitted
value (including empty) is now rejected. Not `optional.Option[string]`,
because `MaxSize` would count slice elements instead of characters.

Fixes #40185

---
AI disclosure: written with Claude Code (claude-opus-5-5) and tested
locally.

---------

Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
Co-authored-by: wxiaoguang <wxiaoguang@gmail.com>
Co-authored-by: silverwind <me@silverwind.io>

modules/optional/option.go

@@ -11,6 +11,8 @@ import "strconv"
 // * non-existing value are represented as an empty slice (None)
 // * existing value is represented as a slice with one element (Some)
 // * multiple values are represented as a slice with multiple elements (Some), the Value is the first element (not well-defined in this case)
+//
+// Attention: Option[T] doesn't work with some web middleware binding rules like "MaxSize" at the moment.
 type Option[T any] []T
 
 func None[T any]() Option[T] {