feat(prefs): per-user UI font scale (A−/A+), saved to the profile

A header A−/value/A+ control scales the whole UI, persisted per user and
restored on login from any booth — cloning the theme-pref pattern end to end.

- DB: users.font_scale (migration 0014; percent, 100 = base, NOT NULL default).
- Server: PUT /api/auth/font-scale (auth-guarded; clamps to 80–160, snaps to a
  10-step); fontScale flows through sessionView → login + /me.
- Client: setFontScalePref + applyFontScale; applied in App alongside theme;
  FontScaleToggle in the header; i18n sq+en.

Scaling uses CSS `zoom` on the root, NOT root font-size: the app's type is pinned
in px (text-[12px] etc., ~230 spots), which a font-size change would not scale —
so the dense Active-sessions / Live-feed logs stayed tiny. `zoom` scales
everything uniformly (text, spacing, icons) like the browser's Ctrl+/−, which is
the readability win for operators who need larger text.

Tests: 4 font-scale auth-route cases (persist + /me, clamp/snap, 400, default-100).
Full workspace build/lint/test green.

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

apps/web/src/api.ts

@@ -73,6 +73,8 @@ export interface SessionUser {
   language: Lang;
   /** Preferred UI theme (loaded from the server on login). */
   theme: Theme;
+  /** Preferred UI font scale, percent of base (100 = base; clamped 80–160). */
+  fontScale: number;
   /** Optional display name (profile metadata); null if unset. */
   fullName: string | null;
   /** Optional contact email (profile metadata); null if unset. */
@@ -105,6 +107,16 @@ export function setThemePref(theme: Theme): Promise<{ theme: Theme }> {
   return apiFetch("/api/auth/theme", { method: "PUT", body: JSON.stringify({ theme }) });
 }
 
+/** Allowed font-scale band (percent of base) + step. The header control clamps to these. */
+export const FONT_SCALE_MIN = 80;
+export const FONT_SCALE_MAX = 160;
+export const FONT_SCALE_STEP = 10;
+
+/** Persist the current user's UI font scale (percent; restored on next login). */
+export function setFontScalePref(fontScale: number): Promise<{ fontScale: number }> {
+  return apiFetch("/api/auth/font-scale", { method: "PUT", body: JSON.stringify({ fontScale }) });
+}
+
 /** Edit MY own profile (display name / email). Returns the refreshed session.
  *  Self-service — touches only the signed-in user; no `user:*` permission needed. */
 export function updateMyProfile(patch: {