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/lib/components/SegmentedButton.svelte

@@ -6,7 +6,7 @@
   {#each options as option}
     <button
       class="rounded-lg font-medium
-        {size === 'small' ? 'px-2.5 py-1 text-[11px]' : 'px-3.5 py-[6px] text-[12px]'}
+        {size === 'small' ? 'px-2.5 py-1 text-[12px]' : 'px-3.5 py-[6px] text-[12px]'}
         {value === option
           ? 'bg-accent text-bg shadow-[0_1px_4px_rgba(214,132,80,0.3)]'
           : 'text-text-secondary hover:text-text hover:bg-hover'}"