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/users/change_password.md

@@ -0,0 +1,39 @@
+# Change Password
+
+Change the current user's password.
+
+**Endpoint**: `PUT /api/users/me/password`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| current_password | string | Yes | Current password |
+| new_password | string | Yes | New password (min 8 chars) |
+
+### Example Request
+
+```json
+{
+  "current_password": "oldPassword",
+  "new_password": "NewSecureP@ss123!"
+}
+```
+
+## Response (204 No Content)
+
+Password changed successfully.
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid input or weak password |
+| 401 | Current password is incorrect |
+| 401 | Invalid or expired token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/developer/api/users/get_profile.md

@@ -0,0 +1,45 @@
+# Get Current User Profile
+
+Retrieve the current authenticated user's profile.
+
+**Endpoint**: `GET /api/users/me`
+**Auth**: Required
+
+## Request Headers
+
+| Header | Type | Required | Description |
+|--------|------|-----------|-------------|
+| Authorization | string | Yes | Bearer token |
+
+### Example Request
+
+```http
+GET /api/users/me
+Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
+```
+
+## Response (200 OK)
+
+```json
+{
+  "id": "uuid",
+  "email": "user@example.com",
+  "username": "john",
+  "first_name": "John",
+  "last_name": "Doe",
+  "theme": "tokyo-night",
+  "role": "user",
+  "max_devices": 10,
+  "created_at": "2026-01-31T10:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 401 | Invalid or expired token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/developer/api/users/update_profile.md

@@ -0,0 +1,50 @@
+# Update Profile
+
+Update the current user's profile information.
+
+**Endpoint**: `PUT /api/users/me/profile`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| first_name | string | No | User's first name |
+| last_name | string | No | User's last name |
+
+### Example Request
+
+```json
+{
+  "first_name": "John",
+  "last_name": "Smith"
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "id": "uuid",
+  "email": "user@example.com",
+  "username": "john",
+  "first_name": "John",
+  "last_name": "Smith",
+  "theme": "tokyo-night",
+  "role": "user",
+  "max_devices": 10,
+  "created_at": "2026-01-31T10:00:00Z"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid input data |
+| 401 | Invalid or expired token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->

docs/developer/api/users/update_theme.md

@@ -0,0 +1,44 @@
+# Update Theme
+
+Update the current user's theme preference.
+
+**Endpoint**: `PUT /api/users/me/theme`
+**Auth**: Required
+**Content-Type**: `application/json`
+
+## Request Body
+
+| Field | Type | Required | Description |
+|--------|------|-----------|-------------|
+| theme | string | Yes | Theme name (e.g., "tokyo-night", "dracula") |
+
+### Example Request
+
+```json
+{
+  "theme": "dracula"
+}
+```
+
+## Response (200 OK)
+
+```json
+{
+  "id": "uuid",
+  "email": "user@example.com",
+  "username": "john",
+  "theme": "dracula",
+  "role": "user"
+}
+```
+
+## Error Responses
+
+| Code | Description |
+|------|-------------|
+| 400 | Invalid theme name |
+| 401 | Invalid or expired token |
+
+## Try It Out
+
+<!-- API Explorer will be inserted here in Phase 3 -->