feat(math): offer the result next to a handwritten = (Issue #46, stage 3)

New module math_assist.rs: when the last two strokes form an = and the pen
rests for 500 ms, the strokes left of it (in a band around the =, up to a
wide gap) are recognized and computed. The suggestion appears next to
the = and shows what was read, e.g. "2⁴ = 16"; tapping it with the pen
or clicking inserts the result as a text box in the ink color and as high
as the handwriting, as one undo step. The chip is not interactive for
egui, so fingers and palms pass through. Any new stroke drops it. In a
chain like x = 2 · 3 = the part after the previous = is computed.

Off by default: ☰ → Input → "Compute formulas after =", saved in
ui.json. math::display writes the recognized expression neatly.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

src/i18n/en.rs

@@ -107,6 +107,7 @@ pub const EN: &[(&str, &str)] = &[
     ("Fehler beim Öffnen: {err}", "Error while opening: {err}"),
     ("Fehler: {err}", "Error: {err}"),
     ("Fehlermeldung: {message}", "Error message: {message}"),
+    ("Formeln nach „=“ ausrechnen", "Compute formulas after “=”"),
     ("Formen (Shape)", "Shapes"),
     ("Formen: Striche werden zu Linien, Rechtecken, Kreisen … (S/F)", "Shapes: strokes become lines, rectangles, circles … (S/F)"),
     ("Fünfeck (Pentagon)", "Pentagon"),
@@ -186,6 +187,7 @@ pub const EN: &[(&str, &str)] = &[
     ("Mindestdruck", "Minimum pressure"),
     ("Mit dem Finger zeichnen", "Draw with finger"),
     ("Mittel", "Medium"),
+    ("Nach einem handgeschriebenen „=“ erscheint das Ergebnis daneben, z. B. 2⁴ = 16. Antippen mit dem Stift oder Klicken fügt es als Text ein.\nGelesen werden Ziffern, + − · : /, Klammern, Hochzahlen, Brüche und x, y, a, b.", "After a handwritten “=” the result appears next to it, e.g. 2⁴ = 16. Tapping it with the pen or clicking inserts it as text.\nDigits, + − · : /, parentheses, exponents, fractions and x, y, a, b are read."),
     ("Name …", "Name …"),
     ("Neon-Gelb", "Neon yellow"),
     ("Netzwerk-Transportfehler: {t}", "Network transport error: {t}"),