ui: Day 3 — global toast system + first error-toast wiring on DictationPage

THE PROBLEM (Codex review + architecture-review.md §1, §14):
Multiple critical paths discard errors silently — `let _ = insert_transcript(...)`,
`.catch(() => {})`, inline `error = ...` state that not every page surfaces. ND
users in particular need explicit feedback when something fails; silent
failure is worse than no feature.

SHIPPED:

src/lib/stores/toasts.svelte.js (new):
- Minimal in-house toast store (~80 lines, no svelte-french-toast dep)
- Severity → palette: info/success → moss, warn → signal, error → ember
- Auto-dismiss durations per severity (success 3s, info 4s, warn 6s,
  error sticky)
- `invokeWithToast(invoke, command, args, errorTitle?)` helper wraps a
  Tauri invoke and toasts on failure while still throwing for callers
  that need to handle the error themselves

src/lib/components/ToastViewport.svelte (new):
- Reads from the toasts store
- Bottom-right stack, max-width clamp on small screens
- aria-live=polite + role=alert on error toasts (screen-reader friendly)
- Slide-in animation honours `html.reduce-motion` for the existing
  preferences toggle
- Severity left-border in brand colours via existing CSS variables

src/routes/+layout.svelte:
- Mounts <ToastViewport /> once at the app root so toasts work from any
  page or window

src/lib/pages/DictationPage.svelte:
- First error-toast wiring: failures starting recording now show a
  sticky toast in addition to the inline `error` panel
- Replaces a previously easy-to-miss failure-mode

NEXT (Day 4 batch):
- Wire toasts into FilesPage, HistoryPage, SettingsPage, ModelDownloader
  (all currently swallow errors)
- Add `add_transcript`, `list_transcripts`, `update_transcript` (closes the
  long-standing TODO Codex flagged), `delete_transcript` Tauri commands
  wrapping the existing storage layer
- Switch addToHistory to dual-write (localStorage + SQLite) so the
  canonical store catches up with the actual transcript flow
- FTS5 transcripts_fts virtual table + search command
- Wrap multi-row writes (decompose_and_store) in DB transactions

src/routes/+layout.svelte

@@ -5,14 +5,17 @@
   import Sidebar from "$lib/Sidebar.svelte";
   import TaskSidebar from "$lib/components/TaskSidebar.svelte";
   import Titlebar from "$lib/components/Titlebar.svelte";
+  import ToastViewport from "$lib/components/ToastViewport.svelte";
   import { page, settings, saveSettings } from "$lib/stores/page.svelte.js";
   import { getPreferences, updatePreferences } from "$lib/stores/preferences.svelte.js";
+  import { hasTauriRuntime } from "$lib/utils/runtime.js";
 
   import { page as sveltePage } from "$app/stores";
 
   let { children } = $props();
 
   const prefs = getPreferences();
+  const tauriRuntimeAvailable = hasTauriRuntime();
 
 
   // Detect secondary windows (float, viewer) — they use +layout@.svelte
@@ -49,6 +52,10 @@
   let hotkeyBackend = $state("unknown"); // "evdev" | "tauri" | "unavailable"
 
   async function initHotkeyBackend() {
+    if (!tauriRuntimeAvailable) {
+      hotkeyBackend = "unavailable";
+      return;
+    }
     try {
       const isWayland = await invoke("is_wayland_session");
       if (isWayland) {
@@ -73,6 +80,7 @@
   }
 
   async function registerGlobalHotkey(hotkey) {
+    if (!tauriRuntimeAvailable) return;
     if (hotkeyBackend === "unknown") return; // not yet initialised
 
     try {
@@ -106,6 +114,7 @@
   // Listen for evdev hotkey events from the Rust backend
   let unlistenEvdev = null;
   async function setupEvdevListener() {
+    if (!tauriRuntimeAvailable) return;
     const { listen } = await import("@tauri-apps/api/event");
     unlistenEvdev = await listen("kon:hotkey-pressed", () => {
       if (page.current !== "dictation") page.current = "dictation";
@@ -161,6 +170,11 @@
     handleResize();
     window.addEventListener("resize", handleResize);
 
+    if (!tauriRuntimeAvailable) {
+      hotkeyBackend = "unavailable";
+      return;
+    }
+
     // Detect and initialise the correct hotkey backend
     await initHotkeyBackend();
 
@@ -177,6 +191,9 @@
 
   onDestroy(() => {
     window.removeEventListener("resize", handleResize);
+    if (!tauriRuntimeAvailable) {
+      return;
+    }
     if (hotkeyBackend === "evdev") {
       invoke("stop_evdev_hotkey").catch(() => {});
     } else if (hotkeyBackend === "tauri" && registeredHotkey) {
@@ -220,3 +237,8 @@
     </div>
   </div>
 {/if}
+
+<!-- Global toast viewport. Mounted once at the app root so any component
+     can call toasts.error(...), toasts.success(...) etc and have it render
+     in the bottom-right of the viewport. (Day 3 of the upgrade plan) -->
+<ToastViewport />