portfolio-edit: form is edit-mode only; submit becomes a + glyph

Two related polishes:

- The add form was auto-shown by the empty-state path so brand-new
  users would see something to act on. That conflicts with the user's
  preference for "Edit always toggles the form, no other path." The
  empty state now shows guidance copy ("click edit to add one")
  instead. exitEditMode always hides the form too.
- The submit "add" word-button is replaced by a square accent-bordered
  + glyph (26×26). Matches the visual weight of the calendar ghost
  next to it but stays in the accent colour so it reads as primary.
  Adds a tiny active-state scale tick for tactile feedback.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/templates/dashboard.html

@@ -84,7 +84,8 @@
           </svg>
         </button>
         <input type="date" id="pf-add-date" class="pf-add__date" hidden>
-        <button type="button" id="pf-add-submit" class="pf-add__submit" disabled>add</button>
+        <button type="button" id="pf-add-submit" class="pf-add__submit"
+                disabled aria-label="Add position" title="Add position">+</button>
       </div>
       <div class="pf-add__notes">
         <span id="pf-add-date-status" class="pf-add-status"></span>