feat(kæde): manglende led forklares i Kræver handling, fanen og som besked

problemText giver årsag og råd for køb/flaskehals/kredsløb og 'slukket af dig'
(userOff-kopi på ADChain.last); modellens problemer får need. Fanen Kæder viser
teksten som rød tekstrække under varen (foldet ud). Kræver handling får én rød
post pr. kæde (højst 2, score 3500+alvor mellem rød tom og rød fuld); klik åbner
fanen med kæden foldet ud. Engangsbesked pr. nyt led (rod|vare|art) kun fra
tick, ikke i opstartsfristen eller på delvise data/ufærdig markscanning.
Manualer og CHANGELOG opdateret.

docs/USER_MANUAL.md

@@ -663,6 +663,24 @@ enable a product in a factory*.
   Other chains carry on. **[Resume]** continues it.
 - At the bottom: *Automatic switches since load*.
 
+#### Missing links (red messages)
+
+If a link in a chain is missing, the mod explains why and what you can do. The text appears in three places:
+
+- **In the Chains tab**: fold the chain out; the reason is shown in red below the red goods row.
+- **In Needs attention**: one red line per chain, for example *Bread is missing a link: Flour*. At most 2 chains
+  are shown; the rest are in the Chains tab. Clicking opens the Chains tab with that chain folded out.
+- **As an on-screen message**: the first time a new link goes missing. The same message only returns after the
+  problem has gone away and come back. No messages during the first 2 minutes after loading or while fields are
+  being scanned.
+
+The messages:
+
+- *None of your factories makes Flour — build one or buy it*: no factory, barn or field supplies the goods.
+- *Mill can only make 800 of 1.0k/mo*: the factory cannot keep up (bottleneck).
+- *Mill (Flour) was turned off by you — turn it on or pause the chain*: you switched the production off yourself.
+- *Loop around Flour — the chain cannot be calculated*: the goods are needed to make themselves.
+
 #### Mark a field free for chains
 
 Open the field in the field guide and switch on **Free for chains (mod may change the crop)**. Only such a