feat(frontend): avatar upload QOL — feedback, re-crop, drop, progress, history

Task 12 batch around the existing crop/preview flow:
- Success Notice (auto-dismiss ~2.5s) after an avatar upload.
- "Reposition" re-crops the current avatar without re-picking a file.
- Drag & drop an image straight onto the profile avatar circle.
- Real upload progress via a new XHR-backed api.upload (fetch has none);
  refreshOnce now also stores the renewed token (reload-restore fix).
- Local avatar history (last 5, IndexedDB, this-device-only) strip that
  re-uploads a stored crop. Stored as ArrayBuffer (Blob is not cloneable
  under structuredClone); label makes the per-device scope explicit.
- Added a "success" tone to Notice.

Infra (same session): registered @shared/@features/@pages/@app/@test path
aliases in vite.config + tsconfigs and rewrote deep ../../../ imports;
moved avatar components into components/avatar/ to keep folders at <=4 files.

Tests: avatarHistory cap/order, recrop visibility, drop-opens-cropper,
client upload/progress/401-retry. bun test 103 pass, lint 0 errors, build ok.

frontend/src/features/landing/sections/explore/HudPlayground.tsx

@@ -1,11 +1,11 @@
 import { useLayoutEffect, useRef, useState } from 'react'
 import { useTranslation } from 'react-i18next'
-import { Button } from '../../../../shared/ui/Button'
-import { themeToCssVars } from '../../../clickgui/themeVars'
-import { parseArgb } from '../../../themes/types'
-import { useActiveTheme } from '../../../themes/useActiveTheme'
+import { Button } from '@shared/ui/Button'
+import { themeToCssVars } from '@features/clickgui/themeVars'
+import { parseArgb } from '@features/themes/types'
+import { useActiveTheme } from '@features/themes/useActiveTheme'
 import { VoxelScene } from '../../effects/VoxelScene'
-import '../../styles/landing.css'
+import '@features/landing/styles/landing.css'
 
 const STEP = 8
 /** Fallback playfield size, used before the box is measured (and under jsdom). */