frontend: add the template configuration UI

The shell: the app mark at both ends of the header, the two top-level
entries between them, and the second-level menu on the right. That menu is
driven entirely by `route.meta.section`, so a route declares which menu it
belongs to and a deep link renders the right one on first paint; routes with
no section (the welcome page) show none.

Screens:
  /                    welcome, with live template and field counts so the
                       page doubles as a connectivity check
  /papers              论文, content still to be decided
  /templates/list      the template table: search, create, edit, preview,
                       delete, batch delete, pagination
  /templates/fields    the field library: the same CRUD, plus level filter

The template form is the centre of it. Fields are chosen from the flat
library by clicking — any subset, any order, the same field more than once —
and the selection is always rendered sorted by `sort`, so editing a number
reorders the outline immediately. Repeats and duplicate sorts are surfaced as
warnings rather than blocked, because a repeat is often deliberate and a tie
is legal; a "自动排序" button renumbers the selection 1..N. The panel on the
right of the picker opens 字段管理 in a second tab, so a half-filled form is
never lost to a navigation.

The preview drawer renders the outline exactly as it will read, at each
field's own size and colour, with the sort value shown alongside — the
quickest way to confirm the ordering before writing against a template.

Also in this change:

  - ApiError now extends Error. It was a plain object, so the common
    `error instanceof Error ? error.message : ...` idiom fell through to a
    generic message and threw away what the server actually said — such as
    which template name is already taken. Server messages now reach the user.
  - font colours are normalised client-side on blur, matching the backend,
    so a hand-typed rgb(255, 0, 0) is tidied up rather than rejected later.
  - api/health.ts is removed: the rewritten welcome page was its only
    consumer, and the counts already prove connectivity.

frontend/src/api/templates.ts

@@ -0,0 +1,85 @@
+import http from './client'
+import type { BatchDeleteResult, PageQuery, PageResult } from './types'
+
+/** One placement of a library field inside a template, with its typography. */
+export interface TemplateField {
+  id: number
+  field_id: number
+  /** Display position. The only thing that decides render order. */
+  sort: number
+  name: string
+  level: number
+  font_size: number
+  font_color: string
+}
+
+/** A template as it appears in the list table. */
+export interface TemplateListItem {
+  id: number
+  name: string
+  abstract: string | null
+  field_count: number
+  created_at: string
+  updated_at: string
+}
+
+/** A template with its outline, already ordered by `sort`. */
+export interface TemplateDetail {
+  id: number
+  name: string
+  abstract: string | null
+  fields: TemplateField[]
+  created_at: string
+  updated_at: string
+}
+
+/** One entry of the selection sent on create/update. */
+export interface TemplateFieldInput {
+  field_id: number
+  sort: number
+}
+
+export interface TemplatePayload {
+  name: string
+  abstract: string | null
+  fields: TemplateFieldInput[]
+}
+
+export async function listTemplates(
+  query: PageQuery = {},
+): Promise<PageResult<TemplateListItem>> {
+  const params: Record<string, unknown> = {}
+  if (query.keyword) params.keyword = query.keyword
+  if (query.page) params.page = query.page
+  if (query.page_size) params.page_size = query.page_size
+
+  const { data } = await http.get<PageResult<TemplateListItem>>('/templates', { params })
+  return data
+}
+
+export async function getTemplate(id: number): Promise<TemplateDetail> {
+  const { data } = await http.get<TemplateDetail>(`/templates/${id}`)
+  return data
+}
+
+export async function createTemplate(payload: TemplatePayload): Promise<TemplateDetail> {
+  const { data } = await http.post<TemplateDetail>('/templates', payload)
+  return data
+}
+
+export async function updateTemplate(
+  id: number,
+  payload: Partial<TemplatePayload>,
+): Promise<TemplateDetail> {
+  const { data } = await http.patch<TemplateDetail>(`/templates/${id}`, payload)
+  return data
+}
+
+export async function deleteTemplate(id: number): Promise<void> {
+  await http.delete(`/templates/${id}`)
+}
+
+export async function batchDeleteTemplates(ids: number[]): Promise<BatchDeleteResult> {
+  const { data } = await http.post<BatchDeleteResult>('/templates/batch-delete', { ids })
+  return data
+}