Add global web nav with project groups and tree sidebar.

Restructure the UI into top-level tabs and a scrollable project tree, and persist optional project groups so demos and settings can organize many projects.

Co-authored-by: Cursor <cursoragent@cursor.com>

internal/sqlite/migrations/003_project_groups.sql

@@ -0,0 +1,12 @@
+CREATE TABLE IF NOT EXISTS project_groups (
+  id         TEXT PRIMARY KEY,
+  name       TEXT NOT NULL,
+  sort_order INTEGER NOT NULL DEFAULT 0,
+  created_at TEXT NOT NULL
+);
+
+CREATE INDEX IF NOT EXISTS idx_project_groups_sort ON project_groups(sort_order, name COLLATE NOCASE);
+
+ALTER TABLE projects ADD COLUMN group_id TEXT REFERENCES project_groups(id) ON DELETE SET NULL;
+
+CREATE INDEX IF NOT EXISTS idx_projects_group ON projects(group_id);