Add app label to root header and auto-reload sexp templates in dev

Show current subdomain name (blog, cart, events, etc.) next to the site
title in the root header row. Remove the redundant second "cart" menu row
from cart overview and checkout error pages.

Add dev-mode hot-reload for sexp templates: track file mtimes and re-read
changed files per-request when RELOAD=true, so .sexp edits are picked up
without restarting services.

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

cart/sexp/sexp_components.py

@@ -511,17 +511,13 @@ async def render_overview_page(ctx: dict, page_groups: list) -> str:
     """Full page: cart overview."""
     main = _overview_main_panel_html(page_groups, ctx)
     hdr = root_header_html(ctx)
-    hdr += render("cart-header-child", inner_html=_cart_header_html(ctx))
     return full_page(ctx, header_rows_html=hdr, content_html=main)
 
 
 async def render_overview_oob(ctx: dict, page_groups: list) -> str:
     """OOB response for cart overview."""
     main = _overview_main_panel_html(page_groups, ctx)
-    oobs = (
-        _cart_header_html(ctx, oob=True)
-        + root_header_html(ctx, oob=True)
-    )
+    oobs = root_header_html(ctx, oob=True)
     return oob_page(ctx, oobs_html=oobs, content_html=main)
 
 
@@ -717,7 +713,6 @@ def _checkout_error_content_html(error: str | None, order: Any | None) -> str:
 async def render_checkout_error_page(ctx: dict, error: str | None = None, order: Any | None = None) -> str:
     """Full page: checkout error."""
     hdr = root_header_html(ctx)
-    hdr += render("cart-header-child", inner_html=_cart_header_html(ctx))
     filt = _checkout_error_filter_html()
     content = _checkout_error_content_html(error, order)
     return full_page(ctx, header_rows_html=hdr, filter_html=filt, content_html=content)