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/lib/pages/DictationPage.svelte

@@ -2,6 +2,7 @@
   import { onMount, onDestroy } from "svelte";
   import { Channel, invoke } from "@tauri-apps/api/core";
   import { page, settings, templates, profiles, addToHistory, addTask, tasks } from "$lib/stores/page.svelte.js";
+  import { toasts } from "$lib/stores/toasts.svelte.js";
   import Card from "$lib/components/Card.svelte";
   import ModelDownloader from "$lib/components/ModelDownloader.svelte";
   import { exportTranscript } from "$lib/utils/export.js";
@@ -342,6 +343,10 @@
       timerInterval = setInterval(updateTimer, 1000);
     } catch (err) {
       error = typeof err === "string" ? err : err?.message || "Microphone error";
+      // Surface the failure as a sticky error toast so it does not get
+      // swallowed if the inline `error` panel is not visible.
+      // (Day 3 of the upgrade plan)
+      toasts.error("Could not start recording", error);
       page.status = "Error";
       page.statusColor = "#e87171";
       cleanup();