Require the household's name for a Regular added from a link (PR #97 review R4-F1)

A pasted link may carry a personal token anywhere in its path, so no part
of it can safely name a dinner: the slug heuristic always let some plain
words through. POST /api/v1/dinners now takes {name} or {name, recipe_url};
a link without a name is 400 invalid_dinner, and nameFromURL is gone. The
link is still kept only for grounding and for the recipe-link duplicate
rule. Name-created Regulars, grounding, and the rename rules are unchanged.

On the Regulars screen, pasting a link shows a "Dinner name" field, and
Add stays disabled until it is filled. The optimistic row shows that name,
never the link.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/server/dinners.go

@@ -19,7 +19,8 @@ const maxDinnerBody = 16 << 10
 // manual recipe of dinners.MaxManualLines lines of each kind.
 const maxGroundBody = 512 << 10
 
-// dinnerCreate is the POST /dinners body: a name or a recipe_url.
+// dinnerCreate is the POST /dinners body: a name, with or without a
+// recipe_url.
 type dinnerCreate struct {
 	Name      *string `json:"name"`
 	RecipeURL *string `json:"recipe_url"`