swarm-ui: header profile menu — initials avatar, authelia settings + logout (#3570)

Adds an "/api/whoami" same-origin nginx proxy to authelia's own
GET /api/user/info (session-cookie authenticated, no swarm-controller
code needed) and a new UserMenu header component: a generated initials
avatar (first letter of display name, coloured from the same seven
base16 chromatic slots the nav accent already cycles through) opening a
popover with the signed-in name, a link to authelia settings, and log
out — both reusing the existing "Authelia" entry from GET /api/links
rather than a second source of the domain.

Per mara's call on the open avatar-mechanism question: initials now,
a real uploaded photo (authelia's settings UI implies pics are
settable) is an explicit future item, not blocking this.

frontend/packages/swarm-ui/src/shell/Shell.tsx

@@ -31,6 +31,7 @@ import type { ComponentChildren } from 'preact';
 import { Link, useLocation } from 'wouter-preact';
 import { LinksMenu } from './LinksMenu.js';
 import { SettingsMenu } from './SettingsMenu.js';
+import { UserMenu } from './UserMenu.js';
 import { useApplyThemeOverride } from '../lib/theme-apply.js';
 import { useApplyMotionOverride } from '../lib/motion-apply.js';
 import './Shell.css';
@@ -284,6 +285,7 @@ export function Shell({ children }: { children: ComponentChildren }) {
         <div class="shell-header-actions">
           <SettingsMenu />
           <LinksMenu />
+          <UserMenu />
         </div>
       </header>
       {/* Keyed by route so it remounts (and replays its entrance