docs(FR-2650): update all locale screenshots to 2× zoom for sharper text rendering (#6891)

Stacked on top of #6868. Resolves part of FR-2650.

## Summary

First incremental batch of locale screenshot recapture under FR-2650. **1 file** (`dashboard.png`) recaptured in 3 locales using `window.switchLanguage()` (PR #5796 / FR-2230).

## Approach

The earlier manifest-based bulk approach (PR #6885, closed) produced too many context-wrong captures (admin vs user view, wrong tabs, modal vs page, drill-down state, etc.). This PR series uses **direct Playwright MCP capture** with per-image context lookup.

Progress tracked at `packages/backend.ai-webui-docs/.agent-output/fr-2650-progress.md` (gitignored, 259 entries pending). Next sessions pick up from there incrementally.

## Capture process used

1. Navigate to `/dashboard` in dev WebUI
2. Hide React Grab dev toolbar
3. `window.switchLanguage('ko')` → screenshot → save to `src/ko/images/dashboard.png`
4. `window.switchLanguage('ja')` → screenshot → save to `src/ja/images/dashboard.png`
5. `window.switchLanguage('th')` → screenshot → save to `src/th/images/dashboard.png`

Viewport 1440×900 logical, light mode, admin user (`admin@lablup.com`).

## Note for reviewers

This PR will be amended/extended as more entries get captured incrementally. Current dashboard layout differs from the prior `en/images/dashboard.png` (different widget arrangement); framing matches the current dev environment which is what the docs render against.

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Author: ironAiken2