feat: add email notifications when a note is shared

Send an email to collaborators when they are added to a shared note.
Uses nodemailer with optional SMTP configuration (auto-enabled when
SMTP_HOST is set). Users can opt out via the notifyOnShare preference.

https://claude.ai/code/session_01P4HSMr7QvKw9TMNikUJD9e

pnpm-lock.yaml

@@ -79,6 +79,9 @@ importers:
       markdown-it:
         specifier: ^14.1.1
         version: 14.1.1
+      nodemailer:
+        specifier: ^8.0.2
+        version: 8.0.2
       svelte-dnd-action:
         specifier: ^0.9.69
         version: 0.9.69(svelte@5.53.7)
@@ -116,6 +119,9 @@ importers:
       '@types/markdown-it':
         specifier: ^14.1.2
         version: 14.1.2
+      '@types/nodemailer':
+        specifier: ^7.0.11
+        version: 7.0.11
       '@types/uuid':
         specifier: ^10.0.0
         version: 10.0.0
@@ -1653,6 +1659,9 @@ packages:
   '@types/node@25.3.5':
     resolution: {integrity: sha512-oX8xrhvpiyRCQkG1MFchB09f+cXftgIXb3a7UUa4Y3wpmZPw5tyZGTLWhlESOLq1Rq6oDlc8npVU2/9xiCuXMA==}
 
+  '@types/nodemailer@7.0.11':
+    resolution: {integrity: sha512-E+U4RzR2dKrx+u3N4DlsmLaDC6mMZOM/TPROxA0UAPiTgI0y4CEFBmZE+coGWTjakDriRsXG368lNk1u9Q0a2g==}
+
   '@types/resolve@1.20.2':
     resolution: {integrity: sha512-60BCwRFOZCQhDncwQdxxeOEEkbc5dIMccYLwbxsS4TUNeVECQ/pBJ0j09mrHOl/JJvpRPGwO9SvE4nR2Nb/a4Q==}
 
@@ -3005,6 +3014,10 @@ packages:
   node-releases@2.0.36:
     resolution: {integrity: sha512-TdC8FSgHz8Mwtw9g5L4gR/Sh9XhSP/0DEkQxfEFXOpiul5IiHgHan2VhYYb6agDSfp4KuvltmGApc8HMgUrIkA==}
 
+  nodemailer@8.0.2:
+    resolution: {integrity: sha512-zbj002pZAIkWQFxyAaqoxvn+zoIwRnS40hgjqTXudKOOJkiFFgBeNqjgD3/YCR12sZnrghWYBY+yP1ZucdDRpw==}
+    engines: {node: '>=6.0.0'}
+
   nyc@18.0.0:
     resolution: {integrity: sha512-G5UyHinFkB1BxqGTrmZdB6uIYH0+v7ZnVssuflUDi+J+RhKWyAhRT1RCehBSI6jLFLuUUgFDyLt49mUtdO1XeQ==}
     engines: {node: 20 || >=22}
@@ -5435,6 +5448,10 @@ snapshots:
     dependencies:
       undici-types: 7.18.2
 
+  '@types/nodemailer@7.0.11':
+    dependencies:
+      '@types/node': 25.3.5
+
   '@types/resolve@1.20.2': {}
 
   '@types/trusted-types@2.0.7': {}
@@ -6770,6 +6787,8 @@ snapshots:
 
   node-releases@2.0.36: {}
 
+  nodemailer@8.0.2: {}
+
   nyc@18.0.0:
     dependencies:
       '@istanbuljs/load-nyc-config': 1.1.0