fix(frontend): show profile popover avatar image (#13155)

### Why / What / How

Why: The new profile popover could show only the user's first initial
even when an avatar image source was provided.

What: This PR updates the account menu avatar layering so the image
renders above the initial fallback, and adds regression coverage for
that behavior.

How: The fallback initial now sits on a lower z-index layer, while
`AvatarImage` is rendered as a positioned higher z-index layer. If no
image source is present, the existing initial fallback behavior is
unchanged.

### Changes 🏗️

- Fix profile popover avatar stacking so the profile image is visible
when `avatarSrc` is provided.
- Add an `InitialAvatar` regression test that verifies the image layer
renders above the fallback initial.
- No configuration changes.

### Checklist 📋

#### For code changes:
- [x] I have clearly listed my changes in the PR description
- [x] I have made a test plan
- [x] I have tested my changes according to the test plan:
- [x] `pnpm --dir autogpt_platform/frontend exec vitest run
src/components/layout/Navbar/components/AccountMenu/components/__tests__/InitialAvatar.test.tsx
src/components/layout/Navbar/components/AccountMenu/__tests__/AccountMenu.test.tsx
--coverage=false`
  - [x] `pnpm --dir autogpt_platform/frontend types`
  - [x] `pnpm --dir autogpt_platform/frontend lint`

<details>
  <summary>Example test plan</summary>
  
  - [ ] Create from scratch and execute an agent with at least 3 blocks
- [ ] Import an agent from file upload, and confirm it executes
correctly
  - [ ] Upload agent to marketplace
- [ ] Import an agent from marketplace and confirm it executes correctly
  - [ ] Edit an agent from monitor, and confirm it executes correctly
</details>

#### For configuration changes:

- [x] `.env.default` is updated or already compatible with my changes
- [x] `docker-compose.yml` is updated or already compatible with my
changes
- [x] I have included a list of my configuration changes in the PR
description (under **Changes**)

<details>
  <summary>Examples of configuration changes</summary>

  - Changing ports
  - Adding new services that need to communicate with each other
  - Secrets or environment variable changes
  - New or infrastructure changes such as databases
</details>