Add Spanish (es) locale with polite usted form

[AdaInTheLab]

Summary

Third and final language addition in this round. German shipped in #59, French in #61, this completes the de/fr/es trio. Spanish, polite usted form throughout.

What's in the locale

• Full mirror of `en.ts` (~250 keys across 30 namespaces) translated to Spanish
• Polite form (`usted`) — appropriate register for an admin tool, neutral across regional variants
• Technical proper nouns kept in English: Steam, EAC, telnet, Cloudflare, Discord, IANA, BBGGRR, etc.
• Game-specific terms use the conventional 7D2D-ES renderings: «Luna de sangre», «Z-Kills», etc.
• Action buttons in standard Spanish UI infinitive form (Guardar, Editar, Eliminar, ...)
• Browser auto-detect extended — visitors with `es-*` locale (es-ES, es-MX, es-AR, etc.) land on Spanish automatically
• broadcastsHint translated to Spanish

Quality note

Same as the German + French PRs: this is a competent first pass, not a professional localization. Common UX surfaces should read naturally; the deeper config-editor field labels are direct translations and would benefit from a domain-fluent native-speaker polish. PRs welcome from your community.

The `usted` form is regionally neutral — works fine for both Latin American and European Spanish-speaking users. If your audience leans heavily one way you might want to revisit specific phrasings, but the default is safe.

Files

File	What
`frontend/src/i18n/locales/es.ts`	New
`frontend/src/i18n/index.ts`	Register `es`, add to `SUPPORTED_LOCALES`, extend browser-language detect

Test plan

• Pull, build, deploy
• Hard-refresh panel → language dropdown shows "Español" between "Français" and "简体中文"
• Switch to Español — sidebar nav, dashboard, settings, etc. render in Spanish
• Hover the language switcher → tooltip shows the broadcasts-not-translated note in Spanish
• Set browser language to `es-ES`, clear localStorage, reload → defaults to Español
• vue-tsc passes (structural typecheck across all 8 locales)

What this round delivered

• de/fr/es with polite-form translations
• Tooltip on language switcher about non-translating broadcasts
• Browser auto-detect picks the user's preferred language
• 8 supported locales total: en, de, fr, es, zh-CN, zh-TW, ja, ko (the latter four still on English placeholders for the keys added during this round — fine to revisit alongside any native-translator pass)

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