fix(crypto): use 27 base-30 chars for Secret Key to prevent data loss

The Secret Key encoder used 26 base-30 characters which can only
represent 30^26 ≈ 2^127.58 values. Since the key is 128 bits,
~25% of generated keys silently lost their high bits during
formatting, making the Emergency Kit key unable to reconstruct
the original bytes on a new browser.

Changed KEY_CHAR_LENGTH from 26 to 27 (30^27 > 2^128). Parser
accepts both old 26-char and new 27-char keys for backward
compatibility. Format: A3-XXXXXX-XXXXXX-XXXXXX-XXXXXX-XXX

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

frontend/src/components/auth/SecretKeyInput.tsx

@@ -103,7 +103,7 @@ export function SecretKeyInput({ value, onChange, error }: SecretKeyInputProps)
     [groups],
   )
 
-  // 26-char key = 4 groups of 6 + 1 group of 2
+  // 27-char key = 4 groups of 6 + 1 group of 3 (old keys: 26 chars, last group = 2)
   const isComplete =
     groups.slice(0, 4).every((g) => g.length === 6) && groups[4].length >= 2
   const hasContent = groups.some((g) => g.length > 0)
@@ -151,7 +151,7 @@ export function SecretKeyInput({ value, onChange, error }: SecretKeyInputProps)
       </div>
       {error && hasContent && !isComplete && (
         <p className="text-xs text-error">
-          Enter all 30 characters of your Secret Key
+          Enter all characters of your Secret Key
         </p>
       )}
     </div>