Jinja2 syntax highlighting + description field + preview toggle

JinjaEditor:
- Custom StreamLanguage parser for Jinja2 syntax highlighting:
  {{ variables }} in blue, {% statements %} in purple, {# comments #} in gray
- Replaced HTML mode (didn't understand Jinja2 syntax)
- Proper monospace font (Consolas/Monaco)

TemplateConfig:
- Added `description` field to model + seed defaults with descriptions
- Description shown on template cards instead of raw template text
- Description input in create/edit form

Preview:
- Toggle behavior: clicking Preview again hides the preview
- Per-slot preview uses preview-raw API (renders current editor content)

i18n: added common.description, templateConfig.descriptionPlaceholder

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

packages/server/src/immich_watcher_server/database/models.py

@@ -119,6 +119,7 @@ class TemplateConfig(SQLModel, table=True):
     id: int | None = Field(default=None, primary_key=True)
     user_id: int = Field(foreign_key="user.id")
     name: str  # e.g. "Default EN", "Default RU"
+    description: str = Field(default="")  # Short description shown on card
     icon: str = Field(default="")
 
     # Event-driven notification templates (full Jinja2)