fix🐛: send datetime fields as RFC3339, not space-separated local time

Both date-pickers -- the search filter and the insert/edit form -- used
value-format="YYYY-MM-DD HH:mm:ss", which formats a picked instant as
e.g. "2026-09-19 12:30:00": no T separator, no offset. dto.go.template
declares every datetime column's InsertReq/UpdateReq field as
time.Time with a plain `json:"..."` tag (R6 leaves that file alone, so
there is no time_format tag to reach for instead), and encoding/json's
default (Un)MarshalJSON for time.Time only accepts RFC3339. The
generated form would submit new and edited datetime values in a shape
Go's JSON decoder cannot parse -- a runtime failure on every create and
update, with nothing in `pnpm type-check` or `pnpm lint` positioned to
see it: both check the request is well-typed TypeScript, not that the
string it produces is a string Go can read.

Changed both to value-format="YYYY-MM-DDTHH:mm:ssZ" -- dayjs's Z token
renders the picker's own local offset, which is what a zero-nanosecond
time.Time (anything without a database column storing sub-second
precision) round-trips to on either side of the wire; confirmed
separately against Go's actual json.Marshal/Unmarshal, not assumed from
the RFC.

The search filter needed the same fix, not just the form: GetPageReq
binds a `time.Time` query field via `form:"..."` (dto.go.template),
and gin's own default for an untagged time.Time binding is also
RFC3339 -- the same failure mode on the query side, one call the
report didn't name but the same root cause reaches.

This is a runtime behaviour change no compiler catches, so it was
verified as one: a Go program exercising encoding/json directly (not
assumed from reading the RFC) confirmed a zero-nanosecond time.Time
marshals to plain RFC3339 with no fractional seconds, and unmarshals
correctly from both a numeric offset and a literal Z. Separately, a
Node script loaded go-admin-ui's own installed dayjs 1.11.21 with the
customParseFormat plugin -- the same plugin element-plus's date-picker
extends dayjs with -- and called the same parseDate path date-picker
panel.mjs uses (time-picker/src/utils.ts, no strict flag passed, so
lenient parsing): formatting with this value-format produced a valid
submission string, and parsing either an offset or a literal-Z string
back with the same format produced a valid, correctly-valued date --
covering create, edit prefill, and update in the two directions that
matter (browser to Go, Go to browser) without needing a live backend.

template/v4/vue.go.template

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