ui: flip tone relabel — "Pro" now maps to INTERMEDIATE, not NOVICE

Reverses the polarity of 71155a6 to match the actual semantics:

- "Novice" stays labelled "Novice" → glossary tooltips, plainer prose.
- "Intermediate" is relabelled "Pro" → terse, assumes fluency, no
  hand-holding. This is the mode an expert reader wants, so the "Pro"
  badge actually fits.

Backend tone values (NOVICE, INTERMEDIATE) are unchanged — no API,
prompt, or stored-preference impact. Only the display strings flip.

Also drops the .tone-toggle button min-width: 10em override added in
71155a6. With "Intermediate" gone from the visible label, the longest
remaining label is "Novice" (6 chars), which fits the shared 5.5em
just like the theme and language toggles.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

app/static/css/layout.css

@@ -157,11 +157,6 @@ body.drawer-open .drawer-backdrop { opacity: 1; }
 .tone-toggle button + button,
 .theme-toggle button + button,
 .lang-toggle button + button { border-left: 1px solid var(--border); }
-/* The tone-toggle's longer option ("Intermediate", 12 chars) needs more
-   room than the shared 5.5em min-width. We size both buttons to fit the
-   longest one so the popup width (set by container width via left/right:0)
-   doesn't get clipped when only the short "Pro" label is active. */
-.tone-toggle button { min-width: 10em; }
 .tone-toggle button:hover,
 .theme-toggle button:hover,
 .lang-toggle button:hover { color: var(--accent); }