Add currency setting and split locale into language + region

Costs rendered as bare numbers because the project had no currency to
render them with. Add one to the profile, beside the existing appearance
preferences:

- users.currency in PocketBase, exposed via /api/me, validated against
  the same 28 codes in the schema, the API and the web app.
- Settings offers the European currencies plus the non-European ones the
  panel already had. Labels come from Intl.DisplayNames rather than a
  hand-kept table, so the lists read in the user's own language and sort
  by what is actually on screen.

The single "Language & region" picker becomes two controls over the one
stored BCP-47 tag, covering European languages and countries, so the two
can be mixed (English in Poland). The API now enforces the
language-REGION shape: the web app feeds the tag straight to Intl, which
throws on a malformed one rather than falling back.

formatKm and the km-count labels passed no locale, so kilometres
followed the browser while the dates and costs beside them followed the
saved preference. They now share one helper that uses the preference.

Rename the "Maintenance log" tab to "Maintenance", the name the
reminder-type list already used.

Amounts are display-only: nothing is converted, so changing currency
reinterprets existing records rather than recalculating them.

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

API Server/internal/api/me.go

@@ -6,6 +6,7 @@ import (
 	"io"
 	"net/http"
 	"net/url"
+	"regexp"
 	"strings"
 	"time"
 
@@ -28,6 +29,7 @@ type userRecord struct {
 	Theme               string `json:"theme"`
 	Locale              string `json:"locale"`
 	DateFormat          string `json:"date_format"`
+	Currency            string `json:"currency"`
 	FontSize            string `json:"font_size"`
 	DeletionRequestedAt string `json:"deletion_requested_at"`
 	Role                string `json:"role"`
@@ -45,6 +47,7 @@ func (rec userRecord) toModel() models.User {
 		Theme:      orDefault(rec.Theme, "system"),
 		Locale:     orDefault(rec.Locale, "en-US"),
 		DateFormat: orDefault(rec.DateFormat, "YMD"),
+		Currency:   orDefault(rec.Currency, "USD"),
 		FontSize:   orDefault(rec.FontSize, "medium"),
 		Role:       orDefault(rec.Role, "user"),
 		Created:    rec.Created,
@@ -90,6 +93,7 @@ type updateMeRequest struct {
 	Theme      *string `json:"theme"`
 	Locale     *string `json:"locale"`
 	DateFormat *string `json:"dateFormat"`
+	Currency   *string `json:"currency"`
 	FontSize   *string `json:"fontSize"`
 }
 
@@ -97,6 +101,23 @@ var validThemes = map[string]bool{"light": true, "dark": true, "system": true}
 var validDateFormats = map[string]bool{"YMD": true, "DMY_NUM": true, "DMY": true, "MDY": true}
 var validFontSizes = map[string]bool{"small": true, "medium": true, "large": true}
 
+// Kept in step with the users.currency select options in setup-pocketbase.mjs:
+// PocketBase rejects anything outside its own list, so accepting a wider set
+// here would only turn a clear 400 into a confusing upstream error.
+var validCurrencies = map[string]bool{
+	"EUR": true, "GBP": true, "CHF": true, "PLN": true, "CZK": true, "HUF": true,
+	"RON": true, "BGN": true, "DKK": true, "SEK": true, "NOK": true, "ISK": true,
+	"ALL": true, "AMD": true, "AZN": true, "BAM": true, "BYN": true, "GEL": true,
+	"MDL": true, "MKD": true, "RSD": true, "RUB": true, "TRY": true, "UAH": true,
+	"USD": true, "CAD": true, "AUD": true, "JPY": true,
+}
+
+// The clients pick language and region separately and join them into this tag,
+// so the stored value is only ever language-REGION. Enforcing that shape here
+// keeps a bad tag out of the record: the web app feeds the locale straight to
+// Intl, which throws on a malformed one rather than falling back.
+var localePattern = regexp.MustCompile(`^[a-z]{2}-[A-Z]{2}$`)
+
 // handleUpdateMe applies a partial update — only fields present in the request
 // body are touched, so the Account/Profile/Appearance sections of the settings
 // panel can each save independently without clobbering the others.
@@ -127,6 +148,10 @@ func (s *Server) handleUpdateMe(w http.ResponseWriter, r *http.Request) {
 		payload["theme"] = *in.Theme
 	}
 	if in.Locale != nil {
+		if !localePattern.MatchString(*in.Locale) {
+			writeError(w, http.StatusBadRequest, "locale must look like en-US")
+			return
+		}
 		payload["locale"] = *in.Locale
 	}
 	if in.DateFormat != nil {
@@ -136,6 +161,13 @@ func (s *Server) handleUpdateMe(w http.ResponseWriter, r *http.Request) {
 		}
 		payload["date_format"] = *in.DateFormat
 	}
+	if in.Currency != nil {
+		if !validCurrencies[*in.Currency] {
+			writeError(w, http.StatusBadRequest, "currency must be a supported ISO 4217 code")
+			return
+		}
+		payload["currency"] = *in.Currency
+	}
 	if in.FontSize != nil {
 		if !validFontSizes[*in.FontSize] {
 			writeError(w, http.StatusBadRequest, "fontSize must be small, medium, or large")