feat: add dev-time feature flag overrides via localStorage (#9075)

## Summary
- Adds `localStorage`-based dev-time override for feature flags, with
`ff:` key prefix (e.g.
`localStorage.setItem('ff:team_workspaces_enabled', 'true')`)
- Override priority: dev localStorage > remoteConfig >
serverFeatureFlags
- Guarded by `import.meta.env.DEV` — tree-shaken to empty function in
production builds
- Extracts `resolveFlag` helper in `useFeatureFlags` to eliminate
repeated fallback pattern

Fixes #9054

## Test plan
- [x] `getDevOverride` unit tests: boolean/number/string/object parsing,
prefix isolation, invalid JSON warning
- [x] `api.getServerFeature` / `serverSupportsFeature` override tests
- [x] `useFeatureFlags` override priority tests, including
`teamWorkspacesEnabled` bypassing guards
- [x] Production build verified: `getDevOverride` compiles to empty
function body, localStorage never accessed
- [x] `pnpm typecheck`, `pnpm lint` clean

┆Issue is synchronized with this [Notion
page](https://www.notion.so/PR-9075-feat-add-dev-time-feature-flag-overrides-via-localStorage-30f6d73d365081b394d3ccc461987b1a)
by [Unito](https://www.unito.io)

src/utils/devFeatureFlagOverride.test.ts

@@ -0,0 +1,60 @@
+import { afterEach, describe, expect, it, vi } from 'vitest'
+
+import { getDevOverride } from '@/utils/devFeatureFlagOverride'
+
+describe('getDevOverride', () => {
+  afterEach(() => {
+    localStorage.clear()
+    vi.restoreAllMocks()
+  })
+
+  it('returns undefined when no override is set', () => {
+    expect(getDevOverride('some_flag')).toBeUndefined()
+  })
+
+  it('returns parsed boolean true', () => {
+    localStorage.setItem('ff:some_flag', 'true')
+    expect(getDevOverride<boolean>('some_flag')).toBe(true)
+  })
+
+  it('returns parsed boolean false', () => {
+    localStorage.setItem('ff:some_flag', 'false')
+    expect(getDevOverride<boolean>('some_flag')).toBe(false)
+  })
+
+  it('returns parsed number', () => {
+    localStorage.setItem('ff:max_upload_size', '209715200')
+    expect(getDevOverride<number>('max_upload_size')).toBe(209715200)
+  })
+
+  it('returns parsed string', () => {
+    localStorage.setItem('ff:some_flag', '"hello"')
+    expect(getDevOverride<string>('some_flag')).toBe('hello')
+  })
+
+  it('returns parsed object', () => {
+    localStorage.setItem('ff:complex', '{"nested": true}')
+    expect(getDevOverride<Record<string, boolean>>('complex')).toEqual({
+      nested: true
+    })
+  })
+
+  it('uses ff: prefix for localStorage keys', () => {
+    localStorage.setItem('some_flag', 'true')
+    expect(getDevOverride('some_flag')).toBeUndefined()
+
+    localStorage.setItem('ff:some_flag', 'true')
+    expect(getDevOverride('some_flag')).toBe(true)
+  })
+
+  it('returns undefined and warns on invalid JSON', () => {
+    const warnSpy = vi.spyOn(console, 'warn').mockImplementation(() => {})
+    localStorage.setItem('ff:bad', 'True')
+
+    expect(getDevOverride('bad')).toBeUndefined()
+    expect(warnSpy).toHaveBeenCalledWith(
+      '[ff] Invalid JSON for override "bad":',
+      'True'
+    )
+  })
+})

src/utils/devFeatureFlagOverride.ts

@@ -0,0 +1,26 @@
+const FF_PREFIX = 'ff:'
+
+/**
+ * Gets a dev-time feature flag override from localStorage.
+ * Stripped from production builds via import.meta.env.DEV tree-shaking.
+ *
+ * Returns undefined (not null) as the "no override" sentinel because
+ * null is a valid JSON value — JSON.parse('null') returns null.
+ * Using undefined avoids ambiguity between "no override set" and
+ * "override explicitly set to null".
+ *
+ * Usage in browser console:
+ *   localStorage.setItem('ff:team_workspaces_enabled', 'true')
+ *   localStorage.removeItem('ff:team_workspaces_enabled')
+ */
+export function getDevOverride<T>(flagKey: string): T | undefined {
+  if (!import.meta.env.DEV) return undefined
+  const raw = localStorage.getItem(`${FF_PREFIX}${flagKey}`)
+  if (raw === null) return undefined
+  try {
+    return JSON.parse(raw) as T
+  } catch {
+    console.warn(`[ff] Invalid JSON for override "${flagKey}":`, raw)
+    return undefined
+  }
+}