feat(i18n): add Slovenian (sl) language support

Closes #580.

Slovenian community contribution from @blazmaric (filed as an issue
with attached files rather than as a PR — files inlined here unchanged
except for the migration number).

## Changes

- **`frontend/src/i18n/locales/sl.json`** — full Slovenian UI
  translations. Covers every top-level key present in `en.json` as
  of pre-CRM beta. The new CRM-module keys (`bills`,
  `businessProfile`, `calendar`, `contracts`, `crm`, `crmDev`,
  `crmSettings`, `dealLineage`, `eventReminderOverride`,
  `hoursLogging`) are not yet translated and will fall back to
  English — same posture as FR / NL / PT / RU / ES currently have
  for the CRM module (see PR #555 description).
- **`frontend/src/components/common/LanguageSelector.tsx`** — adds
  `SLFlag` SVG component + registers `{ code: 'sl', name:
  'Slovenščina', Flag: SLFlag }` in `SUPPORTED_LANGUAGES`. Frontend
  i18n auto-discovers locale files via `import.meta.glob` so no
  separate config registration is needed.
- **`backend/migrations/core/108_seed_sl_email_template_translations.js`** —
  contribution-author's `107_*` filename renumbered to `108_` to
  avoid collision with `107_crm_consolidated.js` that landed on beta
  in the meantime. Idempotent insert via (template_id, language)
  uniqueness check — re-runnable, never overwrites admin edits.
  Covers 17 templates: admin invitation / password reset, archive
  complete, backup completed / failed, customer gallery assigned,
  customer invitation / password reset, database backup completed /
  failed, expiration warning, gallery created / expired, restore
  completed / failed, version update available / test.
- **`backend/src/services/emailProcessor.js`** — adds `.si → sl` to
  the email-domain → language inference map, matching the pattern
  for every other supported locale. A customer with `@example.si`
  now gets Slovenian emails automatically without needing to set
  their preferred_language explicitly.

## Out of scope (consistent with existing locales)

- CRM email templates (quote_sent, invoice_sent, contract_sent, etc.,
  seeded at boot by `crmEmailTemplates.ensureCrmEmailTemplatesSeeded`)
  will fall back to English for Slovenian customers — those seeders
  only emit EN + DE rows today across every locale.
- CRM UI strings under the missing top-level keys listed above will
  fall back to English.

Both gaps mirror the existing FR / NL / PT / RU / ES situation.

frontend/src/components/common/LanguageSelector.tsx

@@ -62,6 +62,22 @@ const ESFlag: React.FC<{ className?: string }> = ({ className = "w-5 h-5" }) =>
   </svg>
 );
 
+const SLFlag: React.FC<{ className?: string }> = ({ className = "w-5 h-5" }) => (
+  <svg className={className} viewBox="0 0 640 480" xmlns="http://www.w3.org/2000/svg">
+    <path fill="#fff" d="M0 0h640v160H0z"/>
+    <path fill="#005DA4" d="M0 160h640v160H0z"/>
+    <path fill="#ED1C24" d="M0 320h640v160H0z"/>
+    <path fill="#005DA4" d="M160 80h120v120c0 60-60 90-60 90s-60-30-60-90V80z"/>
+    <path fill="#fff" d="M172 96h96v100c0 36-28 61-48 74-20-13-48-38-48-74V96z"/>
+    <path fill="#005DA4" d="M184 108h72v82c0 26-18 46-36 59-18-13-36-33-36-59v-82z"/>
+    <path fill="#fff" d="m198 178 22-36 22 36h-44z"/>
+    <path fill="#ED1C24" d="M184 202h72c-6 21-24 37-36 45-12-8-30-24-36-45z"/>
+    <circle fill="#FFD700" cx="198" cy="122" r="5"/>
+    <circle fill="#FFD700" cx="220" cy="116" r="5"/>
+    <circle fill="#FFD700" cx="242" cy="122" r="5"/>
+  </svg>
+);
+
 export const SUPPORTED_LANGUAGES = [
   { code: 'en', name: 'English', Flag: GBFlag },
   { code: 'de', name: 'Deutsch', Flag: DEFlag },
@@ -70,6 +86,7 @@ export const SUPPORTED_LANGUAGES = [
   { code: 'nl', name: 'Nederlands', Flag: NLFlag },
   { code: 'fr', name: 'Français', Flag: FRFlag },
   { code: 'es', name: 'Español', Flag: ESFlag },
+  { code: 'sl', name: 'Slovenščina', Flag: SLFlag },
 ];
 
 export const LanguageSelector: React.FC = () => {