fix(a11y): bump all text below 12px to 12px floor and promote tertiary-on-body to secondary

Brand guidelines (docs/brand/magnotia-brand-guidelines.md §4) say "Never go
below 12px for any text" and "tertiary text must be ≥18px bold or ≥24px
regular". Audit found 246 sub-12px className sites and 166 cases of
text-text-tertiary paired with body sizes.

Bumped text-[9/10/11px] → text-[12px] across 26 .svelte files in src/lib
and src/routes. Promoted text-text-tertiary → text-text-secondary at body
sizes (12-14px) where context allowed; left placeholder pseudo-states,
ternary-conditional inactive states, and line-through "done" states alone
(8 residual pairings, all decorative).

Affects neurodivergent users on 1366×768 budget hardware most directly.
svelte-check: 0 errors, 0 warnings.

src/routes/preview/+page.svelte

@@ -189,7 +189,7 @@
   class="h-full w-full flex flex-col bg-bg text-text p-3 gap-2"
   data-tauri-drag-region
 >
-  <header class="flex items-center justify-between gap-2 text-[11px] text-text-secondary" data-tauri-drag-region>
+  <header class="flex items-center justify-between gap-2 text-[12px] text-text-secondary" data-tauri-drag-region>
     <div class="flex items-center gap-2" data-tauri-drag-region>
       {#if phase === "listening"}
         <span class="inline-block w-[8px] h-[8px] rounded-full bg-text-tertiary animate-pulse"></span>
@@ -214,7 +214,7 @@
     <div class="flex items-center gap-1">
       {#if canRevertToRaw}
         <button
-          class="px-2 py-1 rounded hover:bg-hover text-text-tertiary hover:text-text disabled:opacity-40 flex items-center gap-1 text-[11px]"
+          class="px-2 py-1 rounded hover:bg-hover text-text-secondary hover:text-text disabled:opacity-40 flex items-center gap-1 text-[12px]"
           onclick={revertToRaw}
           disabled={revertBusy}
           title={settings.autoPaste ? "Replace pasted text with raw transcript" : "Put raw transcript on clipboard"}