fix(validation): stop unrecognised boolean strings coercing to a silent false

checkField in packages/validation/src/index.ts (and its browser mirror in
runtime.ts) treated any string other than "true"/"on" as false with no
error, so typos like "treu" or values like "yes"/"1"/"TRUE" silently
passed as false.

Now:
- true/false booleans pass through unchanged
- recognised true strings (case-insensitive, trimmed): true, on, 1, yes
- recognised false strings: false, off, 0, no
- numeric 1/0 coerce (JSON payloads)
- undefined/null/"" still coerce to false (unchecked-checkbox semantics)
- anything else is now a type error (desc.typeMessage or "Must be true or
  false") instead of a silent false

Locked-in behaviours preserved: a required boolean given false still
errors, and parseEnv DEBUG: "true" coercion still works.

Added coverage for recognised strings, numeric 1/0, the type-error
regression guard, absent/empty handling, the required+false case, and a
client/server parity test driving both checkField and the browser runtime
through the same inputs.

Blast radius: searched packages/, examples/, services/ for v.boolean()
usage; all existing call sites (auth consent/rememberDevice, db 'active'
default, example consent checkboxes) feed true/false/'on'/absent values,
none of which change behavior.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

packages/validation/src/index.ts

@@ -114,7 +114,28 @@ export function checkField(
   }
 
   if (desc.type === "boolean") {
-    const value = raw === true || raw === "true" || raw === "on";
+    const empty = raw === undefined || raw === null || raw === "";
+    let value: boolean;
+    if (typeof raw === "boolean") {
+      value = raw;
+    } else if (empty) {
+      value = false;
+    } else if (raw === 1) {
+      value = true;
+    } else if (raw === 0) {
+      value = false;
+    } else if (typeof raw === "string") {
+      const norm = raw.trim().toLowerCase();
+      if (norm === "true" || norm === "on" || norm === "1" || norm === "yes") {
+        value = true;
+      } else if (norm === "false" || norm === "off" || norm === "0" || norm === "no") {
+        value = false;
+      } else {
+        return { value: raw, error: desc.typeMessage ?? "Must be true or false" };
+      }
+    } else {
+      return { value: raw, error: desc.typeMessage ?? "Must be true or false" };
+    }
     if (!desc.optional && !value) {
       return { value, error: desc.requiredMessage || "Required" };
     }