weather: any city he names, not the six in a table (V-421)

The hand-written table understood "какая погода в X" for six values of X.
Ask about Kazan or Tbilisi and the city was dropped silently and answered for
the default location — a correct-sounding answer about the wrong place.

The table is gone. internal/weather already calls Open-Meteo's geocoding
endpoint on every lookup, so the place he named goes straight there and any
place it knows is a place he can ask about. He speaks the prepositional case,
so locationCandidates reverses the two endings that cover most of it: a final
"е" is a nominative "а" or nothing, a final "и" is a soft sign. A wrong
candidate finds no city; it never invents one.

A place the geocoder does not have now reads as "не знаю такого города"
rather than as a provider outage or, worse, as the default city's weather.
ErrLocationUnknown is what carries that apart.

"в" followed by a room or a day word is still the default location. Those
questions are answered by the house sensors and the calendar, not by
Open-Meteo, and they must not be read as a city.

internal/weather/openmeteo.go

@@ -97,7 +97,59 @@ func (p *OpenMeteoProvider) CurrentWeather(ctx context.Context, location string)
 	}, nil
 }
 
+// locationCandidates — the spellings to try for a place taken out of a spoken
+// sentence, in order. He says "какая погода в Казани", so the word arrives in
+// the prepositional case and the geocoder wants the nominative (Vikunja #421).
+//
+// Two cheap reversals cover most of what he says: a final "е" is usually a
+// nominative "а" (Москве → Москва) or nothing at all (Лондоне → Лондон), and a
+// final "и" is usually a soft sign (Казани → Казань). Indeclinable names —
+// Тбилиси, Сочи, Осло — are already nominative and the first candidate answers.
+//
+// Nothing here is a guess about the weather: a wrong candidate finds no city
+// and the caller says so. It only decides which strings are worth asking about.
+func locationCandidates(location string) []string {
+	out := []string{location}
+	add := func(s string) {
+		if s == "" || s == location {
+			return
+		}
+		for _, seen := range out {
+			if seen == s {
+				return
+			}
+		}
+		out = append(out, s)
+	}
+	r := []rune(location)
+	if len(r) < 4 {
+		return out
+	}
+	stem := string(r[:len(r)-1])
+	switch r[len(r)-1] {
+	case 'е', 'Е':
+		add(stem + "а")
+		add(stem)
+	case 'и', 'И':
+		add(stem + "ь")
+		add(stem)
+	case 'у', 'У', 'ю', 'Ю':
+		add(stem + "а")
+	}
+	return out
+}
+
 func (p *OpenMeteoProvider) geocode(ctx context.Context, location string) (lat, lon float64, name string, err error) {
+	for _, cand := range locationCandidates(location) {
+		lat, lon, name, err = p.geocodeOne(ctx, cand)
+		if err == nil {
+			return lat, lon, name, nil
+		}
+	}
+	return 0, 0, "", err
+}
+
+func (p *OpenMeteoProvider) geocodeOne(ctx context.Context, location string) (lat, lon float64, name string, err error) {
 	u := fmt.Sprintf("https://geocoding-api.open-meteo.com/v1/search?name=%s&count=1&language=ru&format=json", url.QueryEscape(location))
 	req, err := http.NewRequestWithContext(ctx, http.MethodGet, u, nil)
 	if err != nil {
@@ -121,7 +173,7 @@ func (p *OpenMeteoProvider) geocode(ctx context.Context, location string) (lat,
 	}
 
 	if len(geo.Results) == 0 {
-		return 0, 0, "", fmt.Errorf("location %q not found", location)
+		return 0, 0, "", fmt.Errorf("%w: %q", ErrLocationUnknown, location)
 	}
 
 	r := geo.Results[0]

internal/weather/openmeteo_test.go

@@ -83,3 +83,29 @@ func TestStubProvider(t *testing.T) {
 		t.Fatalf("StubProvider: want ErrNotConfigured, got %v", err)
 	}
 }
+
+// TestLocationCandidates — he speaks the prepositional case and the geocoder
+// wants the nominative (Vikunja #421).
+func TestLocationCandidates(t *testing.T) {
+	cases := map[string][]string{
+		"Москве":  {"Москве", "Москва", "Москв"},
+		"Казани":  {"Казани", "Казань", "Казан"},
+		"Лондоне": {"Лондоне", "Лондона", "Лондон"},
+		"Тбилиси": {"Тбилиси", "Тбились", "Тбилис"},
+		"Berlin":  {"Berlin"},
+		"Уфе":     {"Уфе"}, // too short to strip — asked as spoken
+	}
+	for in, want := range cases {
+		got := locationCandidates(in)
+		if len(got) != len(want) {
+			t.Errorf("locationCandidates(%q) = %v, want %v", in, got, want)
+			continue
+		}
+		for i := range got {
+			if got[i] != want[i] {
+				t.Errorf("locationCandidates(%q) = %v, want %v", in, got, want)
+				break
+			}
+		}
+	}
+}

internal/weather/weather.go

@@ -7,6 +7,13 @@ import (
 
 var ErrNotConfigured = errors.New("weather: not configured")
 
+// ErrLocationUnknown — the geocoder has no such place. A named city that does
+// not resolve must read differently from a provider outage: one is "I do not
+// know that place", the other is "I could not reach the service", and
+// answering for the default location instead is the defect this replaces
+// (Vikunja #421).
+var ErrLocationUnknown = errors.New("weather: location not found")
+
 type Weather struct {
 	Location    string  `json:"location"`
 	Temperature float64 `json:"temperature"`