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/test/core/auth/secret-storage-info.test.ts

@@ -87,6 +87,22 @@ describe("secretStorageCaveat", () => {
     expect(caveat).toContain("MCP_INSPECTOR_SECRET_KEY");
   });
 
+  it("changes the advice once a passphrase is set but not yet applied", () => {
+    // Telling someone to set MCP_INSPECTOR_SECRET_KEY when they already have
+    // is advice that cannot clear the condition. The condition here is the
+    // pending write, so that is what the sentence names — while the verdict
+    // (still unencrypted, still a warning) stays exactly the same.
+    const pending: SecretStorageInfo = {
+      ...plaintextFile,
+      pendingEncryption: true,
+    };
+    expect(secretStorageTone(pending)).toBe("warn");
+    expect(secretStorageLabel(pending)).toBe("File (unencrypted)");
+    const caveat = secretStorageCaveat(pending);
+    expect(caveat).toContain("next time a secret is saved");
+    expect(caveat).not.toContain("MCP_INSPECTOR_SECRET_KEY");
+  });
+
   it("prefers the memory caveat when a memory store somehow carries the flag", () => {
     // Defensive ordering: `plaintext` is meaningless for memory, and the
     // durability loss is the more consequential of the two statements.

clients/web/src/test/core/react/useInitialConfig.test.tsx

@@ -289,6 +289,27 @@ describe("useInitialConfig", () => {
       ["null", null],
       ["an unknown kind", { kind: "vault", durable: true }],
       ["a missing kind", { durable: true, path: "/x" }],
+      // The dangerous one: `kind` alone passed the old check, and a missing
+      // `plaintext` is falsy, so it rendered as the quiet *encrypted* file —
+      // the most misleading thing this footer can say, produced by the absence
+      // of information rather than by anything the backend claimed.
+      ["a file descriptor with nothing but a kind", { kind: "file" }],
+      [
+        "a file descriptor missing its path",
+        { kind: "file", reason: "fallback", durable: true, plaintext: false },
+      ],
+      [
+        "a file descriptor missing plaintext",
+        { kind: "file", reason: "fallback", durable: true, path: "/x" },
+      ],
+      [
+        "a descriptor with an unknown reason",
+        { kind: "keyring", reason: "vibes", durable: true },
+      ],
+      [
+        "a descriptor with a non-boolean durable",
+        { kind: "memory", reason: "fallback", durable: "no" },
+      ],
     ])("rejects %s", async (_label, value) => {
       const fetchFn = vi
         .fn()