Replace the compact "Add" buttons with a plus icon

The kanban column, sidebar inbox, and add-status forms all had their
own "Add" submit button; with a column-per-status now, that's a lot of
identical labels on screen at once. Replaced with a small square "+"
icon button (.btn-icon), each keeping a descriptive title + aria-label
(dynamic, so it reads "Adding…" while the request is in flight) since
the visible glyph alone isn't an accessible name.

.btn-icon is excluded (":not(.btn-icon)") from the generic submit
button rule and the .field-row/.kanban__new compact-button overrides,
rather than out-specificity'd -- same reasoning as .field's low
specificity against input:focus: it shouldn't depend on source order
which one wins. .kanban__new's own override is now dead (both of its
two buttons are icons) and removed outright.

Left the "Add card" button on the "All tasks" tab as text -- it's a
lone, full-width primary action in a larger form, not one of several
compact icon-sized ones crowded together.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

web/src/components/AppSidebar.vue

@@ -169,7 +169,13 @@ async function onInboxChange(change: ColumnChange) {
         placeholder="New card"
         aria-label="New card"
       />
-      <button type="submit" :disabled="addingToInbox">Add</button>
+      <button
+        type="submit"
+        class="btn-icon"
+        :disabled="addingToInbox"
+        :title="addingToInbox ? 'Adding…' : 'Add card to inbox'"
+        :aria-label="addingToInbox ? 'Adding…' : 'Add card to inbox'"
+      >+</button>
     </form>
   </aside>
 </template>

web/src/components/StatusManager.vue

@@ -175,7 +175,13 @@ useDialog(computed(() => pendingDelete.value !== null), cancelReassign, reassign
         placeholder="Status name"
         aria-label="Status name"
       />
-      <button type="submit" :disabled="addingStatus">{{ addingStatus ? 'Adding…' : 'Add' }}</button>
+      <button
+        type="submit"
+        class="btn-icon"
+        :disabled="addingStatus"
+        :title="addingStatus ? 'Adding…' : 'Add status'"
+        :aria-label="addingStatus ? 'Adding…' : 'Add status'"
+      >+</button>
     </form>
     <p v-if="addStatusError" class="form-error">{{ addStatusError.message }}</p>
   </section>