Add MVP frontend scaffold with FastAPI + HTMX + TailwindCSS

- Created complete frontend structure with Jinja2 templates
- Implemented three main pages: Dashboard, Fleet Roster, and Unit Detail
- Added HTMX auto-refresh for real-time updates (10s interval)
- Integrated dark/light mode toggle with localStorage persistence
- Built responsive card-based UI with sidebar navigation
- Created API endpoints for status snapshot, roster, unit details, and photos
- Added mock data service for development (emit_status_snapshot)
- Implemented tabbed interface on unit detail page (Photos, Map, History)
- Integrated Leaflet maps for unit location visualization
- Configured static file serving and photo management
- Updated requirements.txt with Jinja2 and aiofiles
- Reorganized backend structure into routers and services
- Added comprehensive FRONTEND_README.md documentation

Frontend features:
- Auto-refreshing dashboard with fleet summary and alerts
- Sortable fleet roster table (prioritizes Missing > Pending > OK)
- Unit detail view with status, deployment info, and notes
- Photo gallery with thumbnail navigation
- Interactive maps showing unit coordinates
- Consistent styling with brand colors (orange, navy, burgundy)

Ready for integration with real Series3 emitter data.

backend/routers/photos.py

@@ -0,0 +1,64 @@
+from fastapi import APIRouter, HTTPException
+from fastapi.responses import FileResponse
+from pathlib import Path
+from typing import List
+import os
+
+router = APIRouter(prefix="/api", tags=["photos"])
+
+PHOTOS_BASE_DIR = Path("data/photos")
+
+
+@router.get("/unit/{unit_id}/photos")
+def get_unit_photos(unit_id: str):
+    """
+    Reads /data/photos/<unit_id>/ and returns list of image filenames.
+    Primary photo = most recent file.
+    """
+    unit_photo_dir = PHOTOS_BASE_DIR / unit_id
+
+    if not unit_photo_dir.exists():
+        # Return empty list if no photos directory exists
+        return {
+            "unit_id": unit_id,
+            "photos": [],
+            "primary_photo": None
+        }
+
+    # Get all image files
+    image_extensions = {".jpg", ".jpeg", ".png", ".gif", ".webp"}
+    photos = []
+
+    for file_path in unit_photo_dir.iterdir():
+        if file_path.is_file() and file_path.suffix.lower() in image_extensions:
+            photos.append({
+                "filename": file_path.name,
+                "path": f"/api/unit/{unit_id}/photo/{file_path.name}",
+                "modified": file_path.stat().st_mtime
+            })
+
+    # Sort by modification time (most recent first)
+    photos.sort(key=lambda x: x["modified"], reverse=True)
+
+    # Primary photo is the most recent
+    primary_photo = photos[0]["filename"] if photos else None
+
+    return {
+        "unit_id": unit_id,
+        "photos": [p["filename"] for p in photos],
+        "primary_photo": primary_photo,
+        "photo_urls": [p["path"] for p in photos]
+    }
+
+
+@router.get("/unit/{unit_id}/photo/{filename}")
+def get_photo(unit_id: str, filename: str):
+    """
+    Serves a specific photo file.
+    """
+    file_path = PHOTOS_BASE_DIR / unit_id / filename
+
+    if not file_path.exists() or not file_path.is_file():
+        raise HTTPException(status_code=404, detail="Photo not found")
+
+    return FileResponse(file_path)