fix(tags): tagging works on Android Chrome PWA

TagInput's onkeydown only commits on `e.key === 'Enter' || e.key === ','`.
On Android Chrome with predictive text (GBoard et al), the soft keyboard
keydown fires with `e.key === 'Unidentified'` and `e.keyCode === 229`
during IME composition — the check misses entirely, the Enter never
adds the tag, and the press instead bubbles up to the parent <form>
and submits the whole activity. Result: impossible to tag from mobile.

Fixes:

- Track composition state via oncompositionstart / oncompositionend
  so we ignore commits while the IME is mid-composition.
- Accept Enter via either `e.key === 'Enter'` OR `e.keyCode === 13`
  (and same for comma). preventDefault on every commit attempt so
  the parent form never submits accidentally.
- Add an explicit "Legg til"-button next to the input. Foolproof
  mobile path — a tap is a tap, no IME involved. Desktop users
  still have Enter.
- `enterkeyhint="done"` so the soft keyboard shows a sensible label
  on the action key. autocapitalize/autocorrect/spellcheck off so
  tags don't get mangled by autocomplete suggestions.

Placeholder copy updated to mention the button.

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 +75,12 @@
     }
   }
 
+  /** 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);
+  }
+
   function onType(e: Event) {
     input = (e.target as HTMLInputElement).value;
     clearTimeout(timer);
@@ -87,14 +112,28 @@
   {/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}
+    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">