feat(libra): match income Description field to the expense form

Fold the description guidance into the placeholder and drop the
separate FormDescription helper line, so the income Description field
mirrors the expense form's structure.

src/accounting-app/views/AddIncome.vue

@@ -96,12 +96,11 @@
                   <FormLabel>Description *</FormLabel>
                   <FormControl>
                     <Textarea
-                      placeholder="e.g., Workshop fee, Donation, Service revenue"
+                      placeholder="A detailed description of the income source, and what it was for (event/project/etc)..."
                       v-bind="componentField"
                       rows="3"
                     />
                   </FormControl>
-                  <FormDescription>Describe the source of this income</FormDescription>
                   <FormMessage />
                 </FormItem>
               </FormField>