fix(auth): address Copilot review on the secret-store PR

Six findings, all taken.

- The descriptor reported the *write policy*, not the file. Adding
  `MCP_INSPECTOR_SECRET_KEY` to an install that already has a plaintext
  file flipped `store.encrypted` immediately, so the banner and footer
  claimed "File (encrypted)" while the existing secrets sat readable
  until the next `set`. `FileSecretStore.readOnDiskEncryption()` now
  reads the envelope (never the payload, so it needs no passphrase) and
  the descriptor is built from that. The transitional state gets its own
  `pendingEncryption` flag, which changes the advice without softening
  the verdict — "set a passphrase" is the wrong thing to tell someone who
  already has, so the caveat names the pending write instead.

- The secrets file ignored the configured storage directory. A container
  mounting only that directory was judged unmounted, fell back to
  `memory`, and lost secrets the user had arranged to keep.
  `defaultSecretFilePath()` now follows `MCP_STORAGE_DIR` — the variable
  that already relocates OAuth tokens and `client.json` — between the
  explicit `MCP_INSPECTOR_SECRET_FILE` and the `~/.mcp-inspector`
  default. (The issue comment called it `MCP_INSPECTOR_STORAGE_DIR`; no
  such variable exists.)

- `version` was never checked before the plaintext branch, so a
  `{ version: 2, encryption: "none" }` file was read and silently
  rewritten as version 1, discarding whatever the newer format added —
  the same destroy-to-satisfy-an-additive-request case the encrypted
  branch already refuses. Checked first now, so the answer no longer
  depends on which encryption mode the newer writer chose.

- The doc comment claimed a per-file salt made "a short, memorable value
  safe to use". It does not: the salt defeats precomputation, not
  guessing, and the scrypt cost is deliberately a few milliseconds
  because it runs on every read and write. Both the comment and the
  README now say to use a high-entropy value, generated rather than
  chosen.

- `usableSecretStorage` validated only `kind`, so a partial
  `{ kind: "file" }` was cast through and — because a missing `plaintext`
  is falsy — rendered as the quiet, neutral *encrypted* file. That is the
  confident half-answer the guard exists to reject, produced by the
  absence of information rather than by any claim the backend made.
  `reason`, `durable`, and each kind's own required fields are checked.

- The footer's inline `Tooltip` carried four static props; extracted to a
  `CaveatTooltip` constant per the `.withProps()` rule.

Tests cover each: the policy/file disagreement in both directions and the
no-file-yet fallback, `MCP_STORAGE_DIR` and its precedence against
`MCP_INSPECTOR_SECRET_FILE`, the version refusal (asserting the bytes
survive it) split from the unrecognized-cipher refusal it used to be
conflated with, `readOnDiskEncryption` including the keyless and
unparseable cases, five new rejected descriptor shapes, and the changed
caveat. Full `npm run ci` green.

Signed-off-by: cliffhall <cliff@futurescale.com>

clients/web/src/components/elements/SecretStorageFooter/SecretStorageFooter.tsx

@@ -66,6 +66,15 @@ const FooterDetail = Text.withProps({
   flex: 1,
 });
 
+// `multiline` + a fixed width so the sentence wraps instead of forming a
+// tooltip wider than the modal it explains. Only the `label` is dynamic.
+const CaveatTooltip = Tooltip.withProps({
+  multiline: true,
+  w: 320,
+  position: "top",
+  withArrow: true,
+});
+
 const ICON_SIZE = 14;
 
 /**
@@ -129,11 +138,5 @@ export function SecretStorageFooter({ info }: SecretStorageFooterProps) {
   const tip = [caveat, info.path].filter(Boolean).join(" ");
   if (!tip) return band;
 
-  return (
-    // `multiline` + a width so the sentence wraps instead of forming a tooltip
-    // wider than the modal it explains.
-    <Tooltip label={tip} multiline w={320} position="top" withArrow>
-      {band}
-    </Tooltip>
-  );
+  return <CaveatTooltip label={tip}>{band}</CaveatTooltip>;
 }