Ghost-style the add-card/status icon buttons; convert the last one

- .btn-icon: transparent background, muted icon that turns accent on
  hover -- matching .title-back's treatment exactly, instead of a
  solid accent-filled square.
- The "all tasks" tab's "Add card" button is now the same .btn-icon
  as the kanban/inbox/status ones, so every "add" action in the app
  looks and behaves the same.

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

web/src/views/ProjectView.vue

@@ -211,9 +211,13 @@ async function onCreateInColumn(column: Column) {
             <span>New card</span>
             <input v-model="newText" class="field" type="text" maxlength="1000" required />
           </label>
-          <button type="submit" :disabled="submitting">
-            {{ submitting ? 'Adding…' : 'Add card' }}
-          </button>
+          <button
+            type="submit"
+            class="btn-icon"
+            :disabled="submitting"
+            :title="submitting ? 'Adding…' : 'Add card'"
+            :aria-label="submitting ? 'Adding…' : 'Add card'"
+          >+</button>
         </form>
       </div>