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/pages/FilesPage.svelte

@@ -176,7 +176,7 @@
             Drop audio or video files here
           </p>
         {/if}
-        <p class="text-[11px] text-text-tertiary text-center mt-1.5">
+        <p class="text-[12px] text-text-secondary text-center mt-1.5">
           MP3, WAV, M4A, MP4, FLAC, OGG
         </p>
       </div>
@@ -218,7 +218,7 @@
         ></div>
       </div>
       {#if fileName}
-        <p class="text-[11px] text-text-tertiary mt-1.5">{fileName}</p>
+        <p class="text-[12px] text-text-secondary mt-1.5">{fileName}</p>
       {/if}
     </div>
   {/if}