feat(libra): guide expense account choice + description copy

Step 1 (account selection) now tells the user to pick the "Other"
account if they're not sure. The Description field's placeholder is
rewritten to prompt for a detailed description of the purchase /
invoice / bill and what it was used for (event/project/etc), replacing
the redundant helper line.

Client feedback: with no fixed category taxonomy yet, steering unsure
users to the Other account plus a richer free-text description lets us
reconcile/recategorize later.

Refs aiolabs/webapp#137

src/modules/expenses/components/AddExpense.vue

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