Update plan for 3-photo / 2-ride session flow

Photo 2 serves as both end of ride 1 and start of ride 2.
Server holds in-memory session state across the 3 uploads
and writes both Excel rows after the final photo.

https://claude.ai/code/session_015myTTMs6yDsAGarATe5ePZ

PLAN.md

@@ -3,9 +3,12 @@
 ## How it works
 
 1. Open the web app on your phone
-2. Take a photo of the odometer
-3. Claude Vision API reads the number automatically
-4. The km driven is calculated and saved to an Excel sheet
+2. Take **3 photos** in sequence:
+   - **Photo 1** — odometer at the start of ride 1
+   - **Photo 2** — odometer at the end of ride 1 / start of ride 2
+   - **Photo 3** — odometer at the end of ride 2
+3. Claude Vision API reads each number automatically
+4. Two trip rows are calculated and saved to the Excel sheet
 5. Download the sheet anytime
 
 ---
@@ -23,9 +26,10 @@
 
 ## Excel sheet columns
 
-| Date       | Time  | Odometer (km) | Trip (km) | Notes |
-|------------|-------|---------------|-----------|-------|
-| 2026-03-18 | 08:14 | 84 320        | 47        | Work  |
+| Date       | Time  | Odometer start (km) | Odometer end (km) | Trip (km) | Notes |
+|------------|-------|---------------------|-------------------|-----------|-------|
+| 2026-03-18 | 08:14 | 84 273              | 84 320            | 47        | Ride 1 |
+| 2026-03-18 | 09:05 | 84 320              | 84 391            | 71        | Ride 2 |
 
 ---
 
@@ -34,38 +38,76 @@
 ```
 Driverthing/
 ├── src/
-│   ├── main.rs         # Axum server + routes
+│   ├── main.rs         # Axum server + routes + session state
 │   ├── claude.rs       # Claude API vision call
 │   └── excel.rs        # Excel read/write logic
 ├── templates/
-│   └── index.html      # Mobile camera upload page
+│   └── index.html      # Mobile step-by-step camera upload UI
 ├── Cargo.toml
 └── mileage_log.xlsx    # Generated, gitignored
 ```
 
 ---
 
-## Flow
+## Session state (in-memory)
+
+Between the 3 uploads the server holds a simple session with:
 
 ```
-Phone camera → upload photo
-    → Axum receives multipart/form-data
-        → Claude Vision: "What is the odometer reading?"
-            → Extract number
-                → Calculate delta from last reading
-                    → Append row to Excel
-                        → Show confirmation on screen
+session {
+  reading_1: Option<u32>   # start of ride 1
+  reading_2: Option<u32>   # end of ride 1 / start of ride 2
+  reading_3: Option<u32>   # end of ride 2
+}
+```
+
+When all three readings are present, two rows are written to Excel and the
+session is cleared.
+
+---
+
+## UI flow (single page, steps replace each other)
+
+```
+Step 1: "Take photo of odometer — START of ride 1"
+        [Camera button] → upload
+            ↓ server reads: 84 273 ✓
+
+Step 2: "Take photo of odometer — END of ride 1 / START of ride 2"
+        [Camera button] → upload
+            ↓ server reads: 84 320 ✓
+            ↓ Ride 1: 47 km (shown on screen)
+
+Step 3: "Take photo of odometer — END of ride 2"
+        [Camera button] → upload
+            ↓ server reads: 84 391 ✓
+            ↓ Ride 2: 71 km (shown on screen)
+            ↓ Both rows saved ✓
+
+Done screen: summary + [Download Excel] + [Start new session]
+```
+
+---
+
+## Flow (technical)
+
+```
+POST /upload?step=1  →  read photo → store reading_1 in session
+POST /upload?step=2  →  read photo → store reading_2 → calc ride1 delta → show
+POST /upload?step=3  →  read photo → store reading_3 → calc ride2 delta
+                     →  write 2 rows to Excel → return summary
+GET  /download       →  serve mileage_log.xlsx
 ```
 
 ---
 
 ## Implementation steps
 
-1. Set up Axum server with a multipart file upload endpoint
-2. Send uploaded image (base64) to Claude API with a vision prompt
-3. Parse the odometer number from the response
-4. Read the last recorded odometer value from the Excel file
-5. Calculate the difference (km driven)
-6. Append a new row (date, time, odometer, trip km, notes) to the sheet
-7. Return confirmation to the browser
-8. Add a download endpoint for the Excel file
+1. Set up Axum server with in-memory session state (DashMap or Mutex<HashMap>)
+2. Create a single `/upload` endpoint that accepts `step=1|2|3` + multipart image
+3. Send each uploaded image (base64) to Claude Vision API: "What is the odometer reading in km? Reply with only the number."
+4. Parse the integer from the response and store it in the session
+5. After step 2: calculate ride 1 km, return intermediate confirmation
+6. After step 3: calculate ride 2 km, append both rows to the Excel file, clear session
+7. Mobile-friendly step-by-step HTML UI (progress indicator, camera capture)
+8. `GET /download` endpoint to serve the Excel file