fix(web): editing a library entry warns before it wipes what the console cannot see

`PUT /library/custom/{id}` replaces the whole entry — the host assigns
`slot.prep = input.prep` and `slot.detect = input.detect` outright
(library/custom.rs). But `GET /library` returns a `GameEntry`, which carries
neither field, so the console builds its payload from a read model that has
already lost them. Editing a title to fix a typo silently cleared any prep/undo
commands and detection hints the entry had.

The console cannot round-trip what the read API will not tell it, so this is a
warning, not a fix: the edit form now says plainly that saving replaces the entry
and that anything configured outside the console will be cleared. The actual fix
is host-side — expose `detect` and `prep` on the library read model — and is
noted in the code where it belongs.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

web/messages/de.json

@@ -229,6 +229,7 @@
 	"library_store_steam": "Steam",
 	"library_store_custom": "Eigene",
 	"library_add_title": "Eigenes Spiel hinzufügen",
+	"library_edit_overwrites": "Beim Speichern wird dieser Eintrag durch das ersetzt, was in diesem Formular steht. Vorbereitungs-/Undo-Befehle und Erkennungs-Hinweise, die außerhalb der Konsole gesetzt wurden, erscheinen hier nicht und gehen verloren.",
 	"library_edit_title": "Eigenes Spiel bearbeiten",
 	"library_add_button": "Eigenes Spiel hinzufügen",
 	"library_field_title": "Titel",

web/messages/en.json

@@ -229,6 +229,7 @@
 	"library_store_steam": "Steam",
 	"library_store_custom": "Custom",
 	"library_add_title": "Add a custom game",
+	"library_edit_overwrites": "Saving replaces this entry with what's in this form. Prep/undo commands and detection hints set outside the console aren't shown here and will be cleared.",
 	"library_edit_title": "Edit custom game",
 	"library_add_button": "Add custom game",
 	"library_field_title": "Title",

web/src/sections/Library/GameForm.tsx

@@ -343,6 +343,18 @@ export const GameForm: FC<{
 							help={m.library_field_tags_help()}
 						/>
 					</fieldset>
+					{/* Data-loss warning, not a nicety.
+					`PUT /library/custom/{id}` REPLACES the entry (host: library/custom.rs
+					`update_custom` assigns `slot.prep = input.prep; slot.detect = input.detect`),
+					but `GET /library` returns a `GameEntry`, which carries neither field. So the
+					console cannot round-trip them — anything configured outside this form is dropped
+					by a save it did not intend to touch. The real fix is host-side (expose `detect`
+					and `prep` on the read model); until then, say so before the operator finds out. */}
+					{mode === "edit" && (
+						<p className="rounded-md border border-amber-500/40 bg-amber-500/10 px-3 py-2 text-sm">
+							{m.library_edit_overwrites()}
+						</p>
+					)}
 					{error && (
 						<p
 							role="alert"