feat(kæde): advarsel når en fabrik gården er afhængig af slukkes

Slukker spilleren en produktion, hvis vare et tændt flow kører (manuel vare
eller målet tager den i auto), en stald æder eller en aktiv kæde bruger,
kommer én besked pr. slukning med brugere og lagerdage, og en rød post
'Slukket men nødvendig' i Kræver handling og statusvinduet så længe den er
slukket og nødvendig. Ingen advarsel når en anden tændt produktion dækker,
lageret rækker over 60 dage, kæden selv slukkede den eller den var slukket
ved indlæsning. Tjekket kører i kædens tick også uden kæder; ren logik i
adDependency.lua.

docs/USER_MANUAL.md

@@ -349,8 +349,9 @@ that have something in them:
 - **Fields** — one line per field with field work in progress: *F92 Harvesting · Beet harvester · 45 %*
   (colours as in the Fields tab).
 - **Chains** — only chains with a missing link: *Bread is missing a link: Flour*.
-- **Warnings** — vehicles that have stood still for more than 2 minutes (without Smart Pickup holding them at
-  Wait/outbound) and pending chain suggestions (*Chain: 2 suggestions*).
+- **Warnings** — factories the farm depends on that are switched off (*Switched off but needed: Fermenting
+  silo 3*, see [Warning when a needed factory is switched off](#warning-when-a-needed-factory-is-switched-off)),
+  vehicles that have stood still for more than 2 minutes (without Smart Pickup holding them at Wait/outbound) and pending chain suggestions (*Chain: 2 suggestions*).
 
 At most 18 lines; the rest is summed up as *+k more*. The window is hidden while a menu or dialog is open,
 when the HUD is hidden and while you sleep. Whether it is on is saved per user in
@@ -703,6 +704,33 @@ The messages:
 - *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.
 
+#### Warning when a needed factory is switched off
+
+If you switch off a production the farm depends on, the mod warns you — also when no chain is running. A
+production is needed when one of its goods is used by the farm:
+
+- a running supply flow carries the goods (chosen as goods, or the flow's target takes the goods in auto),
+- a barn is eating the goods right now, or
+- an active chain has the goods as input.
+
+Example: you switch off the fermenting silos, but the flows *Cow Food*, *Goat Food* and *Biogas Silage* fetch
+silage. You then get the message *Warning: you switched off Fermenting silo 3 (Silage) — Cow Food, Goat Food
++1 uses it. Stock lasts about 12 days.* (days from the farm's stock and consumption; left out when unknown).
+The message comes once per switch-off, at most a minute later.
+
+As long as the production is off and still needed, **Needs attention** shows a red line: *Switched off but
+needed: Fermenting silo 3* (at most 2; also in the status window under Warnings). Clicking opens the storage
+tab **Factories** — or **Chains** if only a chain uses the goods.
+
+No warning when:
+
+- another running production makes the goods with enough capacity,
+- the farm has stock for more than 60 days,
+- the chain switched the production off itself, or
+- the production was already off when the game was loaded (then only the Needs attention line is shown).
+
+Productions that no flow, barn or chain uses (for example a bakery without a flow) never give a warning.
+
 #### 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
@@ -814,6 +842,7 @@ every second.
 | *Wheat runs out in … — farm has …, uses …/day* | The farm uses more than it produces, and the good lasts less than 3 days (red below 1 day) |
 | *Milk is 92 % full — full in …* | A produced good is at least 85 % full (red from 95 %) |
 | *Sell Canola now: price is 94 % of the yearly top — …* | The price is at least 90 % of the yearly top, the stock is worth at least 10,000, and the farm does not use it up |
+| *Switched off but needed: Fermenting silo 3* | You switched off a production that a flow, a barn or a chain uses (see [Warning when a needed factory is switched off](#warning-when-a-needed-factory-is-switched-off)); clicking opens the Factories tab |
 
 **Click a line** to act on it:
 
@@ -1317,4 +1346,5 @@ The mod is being tested continuously, and bug reports are very welcome.
 | **Field silo** | A silo near a field used as intermediate storage |
 | **Chain** | The whole path from a production back through factories and barns to the crops in the fields |
 | **Root production** | The production you switch on yourself, which the chain keeps running |
+| **Switched off but needed** | A switched-off production whose goods a flow, a barn or a chain uses — the mod warns once and shows a red line in Needs attention |
 | **Free for chains** | A field marking that lets the mod change the crop automatically |