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

@@ -30,6 +30,13 @@
   let serverHits: { name: string; usage_count: number }[] = $state([]);
   let privateHits: { name: string; count: number }[] = $state([]);
   let timer: ReturnType<typeof setTimeout> | undefined;
+  // True while the soft keyboard is composing (IME predictive text on
+  // Android, hangul/kana/pinyin elsewhere). During composition keydown
+  // fires with e.key === 'Unidentified' and e.keyCode === 229, so a naive
+  // `e.key === 'Enter'` check misses the user's commit-and-press-enter
+  // gesture. We commit on compositionend Enter via the addCurrent button
+  // instead — see the keydown handler.
+  let composing = $state(false);
 
   function add(name: string) {
     const n = name.trim().toLowerCase();
@@ -48,7 +55,19 @@
   }
 
   function onKey(e: KeyboardEvent) {
-    if (e.key === 'Enter' || e.key === ',') {
+    // Detect "commit" intent across desktop and mobile soft keyboards.
+    // Android Chrome with predictive text fires keydown with key
+    // 'Unidentified' + keyCode 229 during composition; Enter committed
+    // afterwards still has keyCode 13 (or 'Enter'), so we accept either.
+    // Comma is the same.
+    if (composing) return;
+    const isEnter = e.key === 'Enter' || e.keyCode === 13;
+    const isComma = e.key === ',' || e.keyCode === 188;
+    if (isEnter || isComma) {
+      // Always preventDefault on Enter — without it the parent <form>
+      // submits the whole activity when the user is just trying to add a
+      // tag (especially noticeable on mobile where the soft keyboard's
+      // "Done" key triggers form submit).
       e.preventDefault();
       add(input);
     } else if (e.key === 'Backspace' && !input && tags.length) {
@@ -56,6 +75,31 @@
     }
   }
 
+  /** Called by the "Legg til"-button. The button is the foolproof mobile
+   *  path — IME composition can swallow Enter, but a tap is a tap. */
+  function addCurrent() {
+    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);
@@ -87,14 +131,29 @@
   {/each}
 </div>
 
-<input
-  type="text"
-  value={input}
-  oninput={onType}
-  onkeydown={onKey}
-  placeholder="Legg til etikett, trykk Enter"
-  aria-label="Ny etikett"
-/>
+<div class="row" style="gap: 0.4rem; align-items: stretch;">
+  <input
+    type="text"
+    value={input}
+    oninput={onType}
+    onkeydown={onKey}
+    onbeforeinput={onBeforeInput}
+    oncompositionstart={() => (composing = true)}
+    oncompositionend={() => (composing = false)}
+    placeholder="Skriv etikett, trykk Enter eller «Legg til»"
+    aria-label="Ny etikett"
+    enterkeyhint="done"
+    autocapitalize="none"
+    autocorrect="off"
+    spellcheck="false"
+    style="flex: 1 1 auto; min-width: 0;"
+  />
+  <button type="button" onclick={addCurrent} disabled={!input.trim()}
+          aria-label="Legg til etikett"
+          style="flex: 0 0 auto;">
+    Legg til
+  </button>
+</div>
 
 {#if serverHits.length || privateHits.length}
   <div class="card" style="margin-top: 0.25rem;" role="listbox" aria-label="Forslag">