chore(webui): drop dead clear() from useSearchQuery, add spec, doc composables

- useSearchQuery.ts: remove clear() — zero callers, since SearchInput's
  clear button already clears by emitting update:modelValue('') through
  v-model, no separate imperative clear needed.
- Add composables/useSearchQuery.spec.ts (3 cases: normalized trims,
  normalized lowercases, hasQuery false for whitespace-only query).
- README.md Structure section: document src/composables/ (useThemeColors,
  useSearchQuery) and mention the new shared SearchInput component
  alongside the existing components/lib entries.

webui/src/composables/useSearchQuery.spec.ts

@@ -0,0 +1,23 @@
+import { describe, expect, it } from 'vitest'
+
+import { useSearchQuery } from './useSearchQuery'
+
+describe('useSearchQuery', () => {
+  it('normalized trims leading/trailing whitespace', () => {
+    const { query, normalized } = useSearchQuery()
+    query.value = '  alice  '
+    expect(normalized.value).toBe('alice')
+  })
+
+  it('normalized lowercases the query', () => {
+    const { query, normalized } = useSearchQuery()
+    query.value = 'AlIcE'
+    expect(normalized.value).toBe('alice')
+  })
+
+  it('hasQuery is false for a whitespace-only query', () => {
+    const { query, hasQuery } = useSearchQuery()
+    query.value = '   '
+    expect(hasQuery.value).toBe(false)
+  })
+})

webui/src/composables/useSearchQuery.ts

@@ -1,24 +1,26 @@
 import { computed, ref } from 'vue'
 
 /**
- * useSearchQuery is the one raw-query → {normalized, hasQuery, clear}
- * adapter every search filter in this panel uses — OverviewView's model
- * search, UsageView's user/group search, ChartsView's scope-picker search
- * all had their own copy of this identical query/normalizedQuery/hasQuery/
- * clearQuery quartet; extracted so it exists once (vue.md: "if you've
- * written it twice, you owe an abstraction"). Callers still own their own
- * matching logic entirely (OverviewView's modelMatches/providerMatches,
+ * useSearchQuery is the one raw-query → {normalized, hasQuery} adapter
+ * every search filter in this panel uses — OverviewView's model search,
+ * UsageView's user/group search, ChartsView's scope-picker search all had
+ * their own copy of this identical query/normalizedQuery/hasQuery
+ * pairing; extracted so it exists once (vue.md: "if you've written it
+ * twice, you owe an abstraction"). Callers still own their own matching
+ * logic entirely (OverviewView's modelMatches/providerMatches,
  * lib/usage-search.ts's helpers) — this only normalizes the raw input the
  * same way every call site already agreed on: trimmed, lowercased.
+ *
+ * No `clear()` here — clearing is just setting `query.value = ''`, and
+ * every caller already gets that for free from SearchInput.vue's own
+ * clear button emitting `update:modelValue('')` through the `v-model`
+ * binding, so a separate clear function would be dead API surface no
+ * caller needs.
  */
 export function useSearchQuery() {
   const query = ref('')
   const normalized = computed(() => query.value.trim().toLowerCase())
   const hasQuery = computed(() => normalized.value.length > 0)
 
-  function clear(): void {
-    query.value = ''
-  }
-
-  return { query, normalized, hasQuery, clear }
+  return { query, normalized, hasQuery }
 }