feat(libra): guide expense description toward categorization

Expand the expense Description helper text to prompt for a detailed
description (what the purchase/invoice/bill was + what it was used for:
event/project/etc) and point users to the "Other" account when none
of the existing accounts match.

Client feedback: with no fixed category taxonomy yet, a richer
description lets us reconcile/recategorize later even when the expense
lands on the Other account.

Refs aiolabs/webapp#137

src/modules/expenses/components/AddExpense.vue

@@ -85,8 +85,7 @@
       <!-- Step 1: Account Selection -->
       <div v-if="currentStep === 1">
         <p class="text-sm text-muted-foreground mb-4">
-          Select the account for this expense. Use the "Other" account if
-          you're not sure.
+          Select the account for this expense
         </p>
         <AccountSelector
           v-model="selectedAccount"
@@ -104,11 +103,16 @@
               <FormLabel>Description *</FormLabel>
               <FormControl>
                 <Textarea
-                  placeholder="A detailed description of the purchase/invoice/bill, and what it was used for (event/project/etc)..."
+                  placeholder="e.g., Biocoop, Ferme des Croquantes, Foix Market, etc"
                   v-bind="componentField"
                   rows="3"
                 />
               </FormControl>
+              <FormDescription>
+                Use the "Other" account if you don't see one that matches. Add a
+                detailed description of the purchase / invoice / bill, and what
+                it was used for (event / project / etc).
+              </FormDescription>
               <FormMessage />
             </FormItem>
           </FormField>