docs: restructure documentation into audience-based portals

BREAKING CHANGE: Documentation URLs have changed

New structure:
- user/ - End-user documentation (device setup, sync guides, frontend)
- developer/ - Developer documentation (API reference, protocols, specs)
- operations/ - Operations documentation (deployment, troubleshooting)
- contributing/ - Contribution guides

Changes:
- Created portal INDEX.md files for each audience section
- Moved device guides to user/devices/ (kobo-setup.md, koreader-setup.md)
- Moved API docs to developer/ (api-reference.md, collections-api.md)
- Moved sync guide to user/sync-guide.md
- Moved troubleshooting to operations/troubleshooting.md
- Moved all split API docs to developer/api/
- Renamed protocol files (kobo-protocol.md, koreader-protocol.md)
- Added placeholder user guides (frontend, user-areas, settings, admin)
- Updated all internal links to new paths
- Updated Go code (http_handler.go, navigation.go) for new paths
- Updated main INDEX.md for audience-based navigation

Benefits:
- Clear separation of user and developer documentation
- Scalable structure for future user guide expansion
- Better organization and discoverability
- Audience-specific landing pages

Related to DOCS_IMPLEMENTATION_PLAN.md Phase 2 completion

docs/developer/api/authentication/login.md

@@ -0,0 +1,49 @@
+# Login User
+
+Authenticate with email and password.
+
+**Endpoint**: `POST /api/auth/login`
+**Auth**: Not required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| email | string | Yes | User's email address |
+| password | string | Yes | User's password |
+
+### Example Request
+
+```json
+{
+  "email": "user@example.com",
+  "password": "SecureP@ss123!"
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...",
+  "refresh_token": "d4f5g6h7...",
+  "user": {
+    "id": "uuid-here",
+    "email": "user@example.com",
+    "username": "john",
+    "role": "user"
+  }
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid email or password |
+| 400 | Missing required fields |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/developer/api/authentication/logout.md

@@ -0,0 +1,35 @@
+# Logout User
+
+Invalidate the current JWT token.
+
+**Endpoint**: `POST /api/auth/logout`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token (e.g., `Bearer eyJhbG...`) |
+
+### Example Request
+
+```http
+POST /api/auth/logout
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (204 No Content)
+
+No response body.
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token |
+| 403 | Token already invalidated |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/developer/api/authentication/refresh_token.md

@@ -0,0 +1,41 @@
+# Refresh Token
+
+Obtain a new JWT token using a refresh token.
+
+**Endpoint**: `POST /api/auth/refresh`
+**Auth**: Not required (uses refresh token)
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| refresh_token | string | Yes | Valid refresh token |
+
+### Example Request
+
+```json
+{
+  "refresh_token": "d4f5g6h7..."
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "token": "new-jwt-token",
+  "refresh_token": "new-refresh-token"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired refresh token |
+| 400 | Missing refresh token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/developer/api/authentication/register.md

@@ -0,0 +1,57 @@
+# Register User
+
+Create a new user account.
+
+**Endpoint**: `POST /api/auth/register`
+**Auth**: Not required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| email | string | Yes | User's email address |
+| username | string | Yes | Desired username |
+| password | string | Yes | Password (min 8 chars) |
+| first_name | string | No | User's first name |
+| last_name | string | No | User's last name |
+
+### Example Request
+
+```json
+{
+  "email": "user@example.com",
+  "username": "john",
+  "password": "SecureP@ss123!",
+  "first_name": "John",
+  "last_name": "Doe"
+}
+```
+
+## Response (201 Created)
+
+```json
+{
+  "token": "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...",
+  "refresh_token": "d4f5g6h7...",
+  "user": {
+    "id": "uuid-here",
+    "email": "user@example.com",
+    "username": "john",
+    "role": "user",
+    "theme": "tokyo-night",
+    "created_at": "2026-01-31T10:00:00Z"
+  }
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid email format, weak password, or missing fields |
+| 409 | Email or username already exists |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->