fix: auto-tagging provider resolution, email test flow, language detection

Auto-tagging:
- CRITICAL FIX: contextual-auto-tag.service.ts was calling getAIProvider()
  (alias for getEmbeddingsProvider) instead of getTagsProvider(). This meant
  auto-tagging used the embeddings provider/model instead of the tags one.
  Now correctly uses getTagsProvider() in both suggestFromExistingLabels and
  suggestNewLabels methods.
- Pass user's detected language to suggestLabels() for localized prompts
  (was hardcoded to 'en')

Email:
- Fix Resend "from" field: pass DB config to sendViaResend() instead of
  re-fetching from DB. Uses SMTP_FROM from config, with localhost-aware fallback.
- Add "Sender email" field in admin Resend section so users can set SMTP_FROM
- Save SMTP_FROM when Resend is selected (was only saved for SMTP mode)
- Test email button now saves config to DB BEFORE testing, so unsaved form
  values are used (was reading stale DB values)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

memento-note/lib/ai/services/contextual-auto-tag.service.ts

@@ -5,7 +5,7 @@
  */
 
 import { prisma } from '@/lib/prisma'
-import { getAIProvider } from '@/lib/ai/factory'
+import { getTagsProvider } from '@/lib/ai/factory'
 import { getSystemConfig } from '@/lib/config'
 
 export interface LabelSuggestion {
@@ -77,7 +77,7 @@ export class ContextualAutoTagService {
 
     try {
       const config = await getSystemConfig()
-      const provider = getAIProvider(config)
+      const provider = getTagsProvider(config)
 
       // Use generateText with JSON response
       const response = await provider.generateText(prompt)
@@ -161,7 +161,7 @@ export class ContextualAutoTagService {
 
     try {
       const config = await getSystemConfig()
-      const provider = getAIProvider(config)
+      const provider = getTagsProvider(config)
 
       // Use generateText with JSON response
       const response = await provider.generateText(prompt)