feat: Make galaxy viewable without login requirement

Implemented public galaxy viewing feature that allows unauthenticated
users to view public thought galaxies via the ?user={did} parameter,
while maintaining privacy controls for node-level visibility.

Changes:
- Updated /api/galaxy/route.ts to support public access:
  * Accept ?user={did} query parameter for viewing specific user's galaxy
  * Show all nodes (including private) for authenticated user viewing own galaxy
  * Filter to only public nodes when viewing someone else's galaxy
  * Return empty state with helpful message when not authenticated
  * Filter links to only show connections between visible nodes

- Added is_public field to database schema:
  * Updated db/schema.surql with DEFAULT true (public by default)
  * Created migration script scripts/add-is-public-field.ts
  * Aligns with ATproto's public-by-default philosophy

- Enhanced ThoughtGalaxy component:
  * Support viewing galaxies via ?user={did} parameter
  * Display user info banner when viewing public galaxy
  * Show appropriate empty state messages based on context
  * Refetch data when user parameter changes

- Created comprehensive Magnitude tests:
  * Test public galaxy viewing without authentication
  * Verify private nodes are hidden from public view
  * Test own galaxy access requires authentication
  * Validate invalid user DID handling
  * Test user info display and navigation between galaxies

- Documented implementation plan in plans/10-public-galaxy-viewing.md

This implements the "public by default" model while allowing future
node-level privacy controls. All canonical data remains on the user's
ATproto PDS, with SurrealDB serving as a high-performance cache.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

db/schema.surql

@@ -67,6 +67,11 @@ DEFINE FIELD coords_3d ON TABLE node TYPE option<array<number>>
     -- Must be NONE or a 3-point array [x, y, z].
     ASSERT $value = NONE OR array::len($value) = 3;
 
+-- Privacy setting: Whether this node is publicly visible.
+-- Defaults to true (public by default, aligns with ATproto philosophy).
+-- When false, node is only visible to the owner.
+DEFINE FIELD is_public ON TABLE node TYPE bool DEFAULT true;
+
 -- Define the vector search index.
 -- We use MTREE (or HNSW) for high-performance k-NN search.
 -- The dimension (3072) MUST match the output of the