Split lib/api.ts by REST domain and add buildSearchParams helper

api.ts had grown to 35 exported symbols in one 432-line file covering
every backend endpoint. Split into webapp/src/lib/api/{shared,logs,
params,metrics,dashboards,alerts,settings}.ts, re-exported from
lib/api/index.ts as a barrel so none of the ~10 importing files needed
their import paths changed (TS/bundler module resolution falls back to
a directory's index.ts when the sibling .ts file is gone).

Also added buildSearchParams to replace fetchLogs/fetchTimeline's
repeated hand-written `if (x !== undefined) search.set(...)` blocks.
Left fetchMetricBucketValues/fetchPromqlSeries's query-building
untouched (byte-for-byte) since the ticket scoped the helper to
fetchLogs/fetchTimeline only.

Reimplemented directly against current main (ticket 089) after the
worktree agent's branch was based on a stale pre-theme-removal state
and reintroduced CSS-upload endpoints main had deliberately deleted.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01TeyhRfMYjMditek6geDRoQ

webapp/src/lib/api/shared.ts

@@ -0,0 +1,103 @@
+// Error shape returned by API handlers on a 4xx/5xx response (see `ErrorDto` on the backend).
+export interface ApiErrorBody {
+  error: string
+}
+
+export class ApiError extends Error {
+  constructor(message: string) {
+    super(message)
+    this.name = 'ApiError'
+  }
+}
+
+// Builds a `URLSearchParams` from an object of optional query fields, skipping any that are
+// `undefined` (falsy strings like `''` are also skipped, matching the hand-written `if` checks
+// this replaces).
+export function buildSearchParams(fields: Record<string, string | number | boolean | undefined>): URLSearchParams {
+  const search = new URLSearchParams()
+  for (const [key, value] of Object.entries(fields)) {
+    if (value !== undefined && value !== '') search.set(key, String(value))
+  }
+  return search
+}
+
+export async function getJson<T>(path: string): Promise<T> {
+  const res = await fetch(path)
+  if (!res.ok) {
+    let message = `${path} failed: ${res.status}`
+    try {
+      const body: unknown = await res.json()
+      if (body && typeof body === 'object' && 'error' in body && typeof (body as { error: unknown }).error === 'string') {
+        message = (body as { error: string }).error
+      }
+    } catch {
+      // body wasn't JSON (or had no `error` field) — fall back to the generic message
+    }
+    throw new Error(message)
+  }
+  return res.json() as Promise<T>
+}
+
+// Throws `ApiError` (not `Error`) with the parsed `{ "error": "..." }` message so callers can surface it inline.
+export async function postJson<T>(path: string, body: unknown): Promise<T> {
+  const res = await fetch(path, {
+    method: 'POST',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(body),
+  })
+  if (!res.ok) {
+    let message = `${path} failed: ${res.status}`
+    try {
+      const errorBody = (await res.json()) as Partial<ApiErrorBody>
+      if (errorBody && typeof errorBody.error === 'string') {
+        message = errorBody.error
+      }
+    } catch {
+      // response body wasn't JSON; fall back to the generic message
+    }
+    throw new ApiError(message)
+  }
+  return res.json() as Promise<T>
+}
+
+// Same error handling as `postJson` (throws `ApiError` with the parsed message) but for a PUT — used where updating in place, rather than creating, is the operation.
+export async function putJson<T>(path: string, body: unknown): Promise<T> {
+  const res = await fetch(path, {
+    method: 'PUT',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(body),
+  })
+  if (!res.ok) {
+    let message = `${path} failed: ${res.status}`
+    try {
+      const errorBody = (await res.json()) as Partial<ApiErrorBody>
+      if (errorBody && typeof errorBody.error === 'string') {
+        message = errorBody.error
+      }
+    } catch {
+      // response body wasn't JSON; fall back to the generic message
+    }
+    throw new ApiError(message)
+  }
+  return res.json() as Promise<T>
+}
+
+export async function sendJson<T>(path: string, method: string, body?: unknown): Promise<T> {
+  const res = await fetch(path, {
+    method,
+    headers: body !== undefined ? { 'Content-Type': 'application/json' } : undefined,
+    body: body !== undefined ? JSON.stringify(body) : undefined,
+  })
+  if (!res.ok) {
+    let message = `${path} failed: ${res.status}`
+    try {
+      const data = await res.json()
+      if (data?.error) message = data.error
+    } catch {
+      // response body wasn't JSON (or was empty) — fall back to the generic message
+    }
+    throw new Error(message)
+  }
+  if (res.status === 204) return undefined as T
+  return res.json() as Promise<T>
+}