phsl / new-api

- Why:
  - Eliminate `gorm.io/datatypes` for a single field and fix scan errors when drivers return JSON as string.
  - Prevent JSONEditor manual mode from locking on invalid JSON and from appending stray characters after “Fill Template”.

- What:
  - Backend (`model/prefill_group.go`):
    - Replaced `datatypes.JSON` with `JSONValue` (based on `json.RawMessage`) for `PrefillGroup.Items`.
    - Implemented `sql.Scanner` and `driver.Valuer` to accept both `[]byte` and `string`.
    - Implemented `MarshalJSON`/`UnmarshalJSON` to preserve raw JSON in API without base64.
    - Converted comments to Chinese.
  - Frontend (`web/src/components/common/ui/JSONEditor.js`):
    - Added `manualText` buffer for manual mode to avoid input being overridden by external value.
    - Only propagate `onChange` when manual text is valid JSON; otherwise show error but do not block typing.
    - Safe manual-mode rendering: derive rows from `manualText` and avoid calling `split` on non-strings.
    - Improved mode toggle: populate `manualText` from visual data; validate before switching back to visual.
    - Fixed “Fill Template” to sync `manualText`, `jsonData`, and `onChange` to avoid stray trailing characters.

- Impact:
  - Resolves: “unsupported Scan, storing driver.Value type string into type *json.RawMessage”.
  - Resolves: `value.split is not a function` in manual mode.
  - Resolves: extra `s` appended after inserting template.
  - API shape and DB column type remain the same (`gorm:"type:json"`); no `go.mod` changes.
  - Lints pass for modified files.

Files changed:
- model/prefill_group.go
- web/src/components/common/ui/JSONEditor.js