refactor(webui): extract repeated dashboard elements into shared components

Markup and helpers that were copied between pages now live in one
place:
- ChartCard, StatDisabledAlert, StatItem, DetailField, BudgetRatioBars,
  LabeledSelect, TargetPicker and LoadStateView components
- lib/scope.ts, lib/columns.ts and lib/chart-palette.ts
- messageOf, isAuthRejection, and the USD to micro-USD conversion
  helpers
- one monthly cost projection, one date formatter, and the
  useCopyFeedback composable

Behaviour is unchanged. Before and after screenshots of every page
match at 390 and 1440 px. Empty-state text now comes from the shared
EmptyState component, and the user run-out date uses the short date
format. admin_assets_gen.go is regenerated.

Claude-Session: https://claude.ai/code/session_01V5jYJNxpDFdJkyyExpADrv

webui/src/composables/useTargetPicker.spec.ts

@@ -0,0 +1,47 @@
+import { describe, expect, it } from 'vitest'
+
+import { useTargetPicker } from './useTargetPicker'
+
+describe('useTargetPicker', () => {
+  it('defaults to targetKind "group" with an empty targetName', () => {
+    const picker = useTargetPicker(() => ['eng'], () => ['alice'])
+    expect(picker.targetKind.value).toBe('group')
+    expect(picker.targetName.value).toBe('')
+  })
+
+  it('targetNames reads groupNames() while targetKind is "group", userNames() while "user"', () => {
+    const picker = useTargetPicker(() => ['eng', 'ops'], () => ['alice', 'bob'])
+    expect(picker.targetNames.value).toEqual(['eng', 'ops'])
+    picker.targetKind.value = 'user'
+    expect(picker.targetNames.value).toEqual(['alice', 'bob'])
+  })
+
+  it('nameValid applies isValidName for a group target', () => {
+    const picker = useTargetPicker(() => [], () => [])
+    picker.targetName.value = 'valid-name.1'
+    expect(picker.nameValid.value).toBe(true)
+    picker.targetName.value = 'has spaces'
+    expect(picker.nameValid.value).toBe(false)
+  })
+
+  it('nameValid applies isValidUserName (non-empty only) for a user target', () => {
+    const picker = useTargetPicker(() => [], () => [])
+    picker.targetKind.value = 'user'
+    picker.targetName.value = 'has spaces are fine for a user'
+    expect(picker.nameValid.value).toBe(true)
+    picker.targetName.value = '   '
+    expect(picker.nameValid.value).toBe(false)
+  })
+
+  it('snippetLabel names middleware.yaml for a group target', () => {
+    const picker = useTargetPicker(() => [], () => [])
+    expect(picker.snippetLabel.value).toBe('Paste into middleware.yaml')
+  })
+
+  it('snippetLabel names the user\'s users.json line, with the current target name, for a user target', () => {
+    const picker = useTargetPicker(() => [], () => [])
+    picker.targetKind.value = 'user'
+    picker.targetName.value = 'alice'
+    expect(picker.snippetLabel.value).toBe('Merge into alice’s users.json line')
+  })
+})