feat(preferences): configurable photo info attributes stored in user profile

- backend: users.preferences jsonb (migration 000004), Me returns preferences, PATCH /auth/me/preferences with validation (visible list + labels)
- frontend: gear in info section opens PhotoInfoSettingsDialog; base attribute set shown by default, remaining under ellipsis; per-attribute visibility and custom labels persisted to the profile; reset clears settings from the profile
- frontend: dialog has select-all/deselect-all icon buttons (no 'show all'); EXIF constants extracted to lib/exif.ts
- fix: render modal overlays via portal to document.body so they center in the viewport despite transformed panel/rail ancestors
- openspec: sync lightroom-workspace-ui artifacts (spec/design/proposal/tasks 9.8)

backend/internal/auth/handlers.go

@@ -1,6 +1,9 @@
 package auth
 
 import (
+	"encoding/json"
+	"fmt"
+	"io"
 	"net/http"
 
 	"github.com/gin-gonic/gin"
@@ -89,5 +92,79 @@ func (h *Handler) Me(c *gin.Context) {
 		httpx.RespondError(c, httpx.Unauthorized("unauthorized", "user not found"))
 		return
 	}
-	c.JSON(http.StatusOK, gin.H{"user": u.Public()})
+	prefs := u.Preferences
+	if prefs == nil {
+		prefs = map[string]any{}
+	}
+	c.JSON(http.StatusOK, gin.H{"user": u.Public(), "preferences": prefs})
+}
+
+const (
+	maxVisibleAttrs       = 500
+	maxLabelEntries       = 500
+	maxPreferenceKeyLen   = 128
+	maxPreferenceValueLen = 128
+)
+
+type photoInfoPreferences struct {
+	Visible []string          `json:"visible"`
+	Labels  map[string]string `json:"labels,omitempty"`
+}
+
+type preferencesPayload struct {
+	PhotoInfo *photoInfoPreferences `json:"photo_info,omitempty"`
+}
+
+func (p preferencesPayload) validate() error {
+	pi := p.PhotoInfo
+	if pi == nil {
+		return nil
+	}
+	if len(pi.Visible) > maxVisibleAttrs {
+		return fmt.Errorf("too many visible attributes")
+	}
+	for _, k := range pi.Visible {
+		if k == "" || len(k) > maxPreferenceKeyLen {
+			return fmt.Errorf("invalid visible attribute key")
+		}
+	}
+	if len(pi.Labels) > maxLabelEntries {
+		return fmt.Errorf("too many label overrides")
+	}
+	for k, v := range pi.Labels {
+		if k == "" || len(k) > maxPreferenceKeyLen {
+			return fmt.Errorf("invalid label key")
+		}
+		if len(v) > maxPreferenceValueLen {
+			return fmt.Errorf("label value too long")
+		}
+	}
+	return nil
+}
+
+func (h *Handler) UpdatePreferences(c *gin.Context) {
+	body, err := io.ReadAll(io.LimitReader(c.Request.Body, 1<<20))
+	if err != nil {
+		httpx.RespondError(c, httpx.BadRequest("invalid_preferences", "could not read request body"))
+		return
+	}
+	var payload preferencesPayload
+	if err := json.Unmarshal(body, &payload); err != nil {
+		httpx.RespondError(c, httpx.BadRequest("invalid_preferences", "preferences must be a JSON object"))
+		return
+	}
+	if err := payload.validate(); err != nil {
+		httpx.RespondError(c, httpx.BadRequest("invalid_preferences", err.Error()))
+		return
+	}
+	raw, err := json.Marshal(payload)
+	if err != nil {
+		httpx.RespondError(c, httpx.BadRequest("invalid_preferences", "could not encode preferences"))
+		return
+	}
+	if err := h.svc.store.UpdatePreferences(c.Request.Context(), UserID(c), raw); err != nil {
+		httpx.RespondError(c, err)
+		return
+	}
+	c.JSON(http.StatusOK, gin.H{"preferences": payload})
 }