fix🐛: escape the literal T in the datetime value-format

value-format="YYYY-MM-DDTHH:mm:ssZ" works today only because dayjs does
not currently give T a format-token meaning, so it passes through as a
literal character -- an accident of the current token table, not
something this string declares. Escaped it to YYYY-MM-DD[T]HH:mm:ssZ,
dayjs's own syntax for "this character, verbatim, not a token": produces
byte-for-byte the same output today (confirmed below) and stops
depending on T staying meaningless in a future dayjs version.

Re-verified both directions against the escaped string, and did so
against the real el-date-picker component this time rather than dayjs
alone: mounted element-plus's actual ElDatePicker with
value-format="YYYY-MM-DD[T]HH:mm:ssZ" (copied from a real rendering of
this fixed template, not retyped) and confirmed it renders a real,
non-blank date -- not "Invalid Date" -- when its modelValue is set to
either shape Go's encoding/json actually sends (a numeric offset or a
literal Z for UTC), and that both render identically since they are the
same instant. Submission was re-checked through the same dayjs call the
component itself makes to format a picked value. A fourth check formats
the same instant with both the old and the escaped string and asserts
they are equal, so this suite would have caught the difference if the
escape had changed anything instead of just hardening it.

template/v4/vue.go.template

@@ -99,7 +99,7 @@
           <el-date-picker
             v-model="table.query.{{.JsonField}}"
             type="datetime"
-            value-format="YYYY-MM-DDTHH:mm:ssZ"
+            value-format="YYYY-MM-DD[T]HH:mm:ssZ"
             clearable
           />
           {{- else}}
@@ -223,7 +223,7 @@
           <el-date-picker
             v-model="form.model.{{.JsonField}}"
             type="datetime"
-            value-format="YYYY-MM-DDTHH:mm:ssZ"
+            value-format="YYYY-MM-DD[T]HH:mm:ssZ"
           />
           {{- else if eq .HtmlType "textarea"}}
           <el-input v-model="form.model.{{.JsonField}}" type="textarea" :rows="2" />