Add planner landing page, archive change

Replace blank home page with landing page: hero with "Start Planning"
CTA, 5 feature cards, Journal CTA, footer with attribution. All strings
in en + de. Sync planner-session spec and archive change.

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

openspec/specs/planner-session/spec.md

@@ -73,6 +73,21 @@ The Planner SHALL display presence indicators showing which users are currently
 - **WHEN** a user disconnects from the session
 - **THEN** their cursor disappears from all other participants' maps within 5 seconds
 
+### Requirement: Planner home page
+The Planner home page SHALL explain the tool's purpose and provide a one-click way to start planning.
+
+#### Scenario: First-time visitor
+- **WHEN** a user visits planner.trails.cool for the first time
+- **THEN** they see a landing page explaining collaborative route planning, key features, and a prominent "Start Planning" button
+
+#### Scenario: Start a session
+- **WHEN** a user clicks "Start Planning"
+- **THEN** a new anonymous session is created and the user is redirected to the session view
+
+#### Scenario: Journal link
+- **WHEN** a user wants to save routes permanently
+- **THEN** a secondary CTA links to trails.cool for account creation
+
 ### Requirement: No user data collection
 The Planner SHALL NOT collect, store, or track any personal user data. Sessions are anonymous by default.