fix(tags): Firefox Android — Enter to commit now works

The prior fix (a94a380) handled Chrome Android via the keydown path.
Firefox Android does something different: the soft-keyboard Enter is
intercepted at the input layer as "advance to next form field" — no
keydown fires for us to intercept, so the user's tag was lost and
focus jumped to the next ActivityForm input.

Add an onbeforeinput handler. InputEvent.inputType === 'insertLineBreak'
fires on every modern browser when the user submits via Enter,
even when keydown is suppressed at the IME/keyboard layer.
preventDefault'ing here cancels the focus-advance before it happens.

Idempotent with the existing onKey:
  - Chrome: keydown fires with Enter, our preventDefault cancels the
    line-break, so beforeinput never fires.
  - Firefox Android: no keydown for Enter, beforeinput fires with
    insertLineBreak, we commit there.
  - Android Chrome with IME composition: composing flag short-circuits
    both paths so we don't commit mid-composition.

Comma + button remain as platform-agnostic fallbacks.

frontend/src/components/TagInput.svelte

@@ -81,6 +81,25 @@
     add(input);
   }
 
+  /**
+   * beforeinput fires before the browser commits a value change. We use it
+   * to catch Enter on platforms where keydown doesn't deliver a usable
+   * Enter event — notably Firefox on Android, which treats the soft-
+   * keyboard Enter as "advance to next form field" at the input layer
+   * (no keydown for us to intercept). InputEvent.inputType === 'insertLineBreak'
+   * still fires here, so we get to preventDefault before focus moves.
+   * Idempotent with onKey: keydown's preventDefault on Chrome cancels
+   * the would-be insertion, so beforeinput never fires there.
+   */
+  function onBeforeInput(e: Event) {
+    const ie = e as InputEvent;
+    if (composing) return;
+    if (ie.inputType === 'insertLineBreak') {
+      e.preventDefault();
+      add(input);
+    }
+  }
+
   function onType(e: Event) {
     input = (e.target as HTMLInputElement).value;
     clearTimeout(timer);
@@ -118,6 +137,7 @@
     value={input}
     oninput={onType}
     onkeydown={onKey}
+    onbeforeinput={onBeforeInput}
     oncompositionstart={() => (composing = true)}
     oncompositionend={() => (composing = false)}
     placeholder="Skriv etikett, trykk Enter eller «Legg til»"