URL restructure, 404 page, trailing slash normalization, layout fixes

- Rename /reactive-islands/ → /reactive/, /reference/ → /hypermedia/reference/,
  /examples/ → /hypermedia/examples/ across all .sx and .py files
- Add 404 error page (not-found.sx) working on both server refresh and
  client-side SX navigation via orchestration.sx error response handling
- Add trailing slash redirect (GET only, excludes /api/, /static/, /internal/)
- Remove blue sky-500 header bar from SX docs layout (conditional on header-rows)
- Fix 405 on API endpoints from trailing slash redirect hitting POST/PUT/DELETE
- Fix client-side 404: orchestration.sx now swaps error response content
  instead of silently dropping it
- Add new plan files and home page component

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/app.py

@@ -95,6 +95,8 @@ def create_app() -> "Quart":
         ),
     }
 
+    app.url_map.strict_slashes = False
+
     from sxc.pages import setup_sx_pages
     setup_sx_pages()
 
@@ -104,6 +106,60 @@ def create_app() -> "Quart":
     from shared.sx.pages import auto_mount_pages
     auto_mount_pages(app, "sx")
 
+    @app.before_request
+    async def trailing_slash_redirect():
+        from quart import request, redirect
+        path = request.path
+        if (path != "/"
+                and not path.endswith("/")
+                and request.method == "GET"
+                and not path.startswith(("/static/", "/internal/", "/auth/"))
+                and "/api/" not in path
+                and "." not in path.rsplit("/", 1)[-1]):
+            qs = request.query_string.decode()
+            target = path + "/" + ("?" + qs if qs else "")
+            return redirect(target, 301)
+
+    @app.errorhandler(404)
+    async def sx_not_found(e):
+        from quart import request, make_response
+        from shared.browser.app.utils.htmx import is_htmx_request
+        from shared.sx.jinja_bridge import get_component_env, _get_request_context
+        from shared.sx.async_eval import async_eval_slot_to_sx
+        from shared.sx.types import Symbol, Keyword
+        from shared.sx.helpers import full_page_sx, oob_page_sx, sx_response
+        from shared.sx.pages import get_page_helpers
+        from shared.sx.page import get_template_context
+
+        path = request.path
+        content_ast = [
+            Symbol("~sx-doc"), Keyword("path"), path,
+            [Symbol("~not-found-content"), Keyword("path"), path],
+        ]
+
+        env = dict(get_component_env())
+        env.update(get_page_helpers("sx"))
+        ctx = _get_request_context()
+
+        try:
+            content_sx = await async_eval_slot_to_sx(content_ast, env, ctx)
+        except Exception:
+            from shared.browser.app.errors import _sx_error_page
+            html = _sx_error_page("404", "NOT FOUND",
+                                  image="/static/errors/404.gif")
+            return await make_response(html, 404)
+
+        if is_htmx_request():
+            return sx_response(
+                await oob_page_sx(content=content_sx),
+                status=404,
+            )
+        else:
+            tctx = await get_template_context()
+            html = await full_page_sx(tctx, header_rows="",
+                                      content=content_sx)
+            return await make_response(html, 404)
+
     return app