Sheet: clarify that editing Level inline doesn't recompute HP/slots (use Level Up)

The inline Level field silently changed only proficiency-derived stats, leaving HP and
spell slots at the old level with no signal. Adds a hint pointing to the Level Up flow,
which applies HP/slots correctly. (Auto-recompute is intentionally avoided since HP/slots
are often hand-tuned.)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/features/characters/CharacterSheet.tsx

@@ -179,6 +179,7 @@ export function CharacterSheet({ character }: { character: Character }) {
         </Labeled>
         <Labeled label="Level">
           <NumberField value={c.level} min={1} max={20} onChange={(level) => update({ level })} />
+          <p className="mt-1 text-[10px] leading-tight text-muted">Direct edits update proficiency-based stats only — HP and spell slots aren’t recalculated. Use <span className="text-ink">Level Up</span> to advance a level.</p>
         </Labeled>
         <Labeled label="Speed">
           <NumberField value={c.speed} min={0} onChange={(speed) => update({ speed })} />