Add an info box so the non-linear nature of desired retention can be seen

ftl/core/deck-config.ftl

@@ -390,6 +390,12 @@ deck-config-compute-optimal-retention-tooltip =
     or too high for the amount of cards you're trying to learn. This number can be useful as a reference, but it
     is not recommended to copy it into the desired retention field.
 deck-config-please-save-your-changes-first = Please save your changes first.
+deck-config-a-100-day-interval = 
+    { $days ->
+        [one] A 100 day interval will become { $days } day.
+       *[other] A 100 day interval will become { $days } days.
+    }
+
 deck-config-wait-for-audio = Wait for audio
 deck-config-show-reminder = Show Reminder
 deck-config-answer-again = Answer Again

ts/deck-options/FsrsOptions.svelte

@@ -43,6 +43,7 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
     }
     $: computing = computingWeights || checkingWeights || computingRetention;
     $: customSearch = `preset:"${$presetName}"`;
+    $: desiredRetentionWarning = getRetentionWarning($config.desiredRetention);
 
     let computeRetentionProgress:
         | ComputeWeightsProgress
@@ -58,6 +59,14 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
     $: if (optimalRetentionRequest.daysToSimulate > 3650) {
         optimalRetentionRequest.daysToSimulate = 3650;
     }
+
+    function getRetentionWarning(retention: number): string {
+        const days = Math.round(9 * 100 * (1.0 / retention - 1.0));
+        if (days === 100) {
+            return "";
+        }
+        return tr.deckConfigA100DayInterval({ days });
+    }
     async function computeWeights(): Promise<void> {
         if (computingWeights) {
             await setWantsAbort({});
@@ -219,6 +228,8 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
     </SettingTitle>
 </SpinBoxFloatRow>
 
+<Warning warning={desiredRetentionWarning} className="alert-info" />
+
 <SpinBoxFloatRow
     bind:value={$config.sm2Retention}
     defaultValue={defaults.sm2Retention}

ts/deck-options/Warning.svelte

@@ -9,11 +9,12 @@ License: GNU AGPL, version 3 or later; http://www.gnu.org/licenses/agpl.html
     import Row from "../components/Row.svelte";
 
     export let warning: string;
+    export let className = "alert-warning";
 </script>
 
 {#if warning}
     <Row>
-        <div class="col-12 alert alert-warning mb-0" in:slide out:slide>
+        <div class="col-12 alert {className} mb-0" in:slide out:slide>
             {withoutUnicodeIsolation(warning)}
         </div>
     </Row>