feat(auth): file-backed and in-memory SecretStore for hosts with no keychain

Closes #1950

Before this, a box with no reachable OS keychain — the published
container (no D-Bus session, #1848), Android/Termux (no platform binary,
#1905), a minimal Linux install without libsecret — could not persist an
OAuth client secret or a stdio `env:` value at all. `KeyringSecretStore`
degraded into a store that silently holds nothing: `get` returned null,
`delete` no-opped, and `set` hard-failed with a 503. That is the correct
failure behavior, but it left the documented, supported way to run the
Inspector with no secret persistence whatsoever.

Adds the two missing implementations and, more importantly, the policy
that picks one and the surfaces that say which was picked.

- `FileSecretStore` — one `0600` JSON document at
  `~/.mcp-inspector/secrets.json`, encrypted with AES-256-GCM whenever
  `MCP_INSPECTOR_SECRET_KEY` is set (scrypt, per-file random salt). The
  whole map is encrypted as a unit rather than value-by-value so the
  account names — `${serverId}:${field}` — are hidden too; a per-value
  scheme would leave a readable index of which servers you hold a client
  secret for. A file that can no longer be decrypted reads as empty
  (matching the interface's read tolerance) but *refuses to be written*,
  because replacing a file of still-valid secrets to satisfy an additive
  request destroys data.

- `secret-store-selection.ts` — the policy.
  `MCP_INSPECTOR_SECRET_STORE=keyring|file|memory` wins outright;
  otherwise probe the keychain and, if unreachable, fall back loudly. The
  fallback is `memory` in a container whose secrets directory is not on a
  mount, and `file` otherwise — so mounting the volume the README already
  recommends for the catalog flips the same run to durable storage with
  no configuration, and an unmounted container gets the honest answer
  rather than a file `--rm` will discard. Cached per process so the
  banner, `/api/config`, and the store doing the writing cannot disagree.

- Automatic, but never silent. The downgrade prints at startup, rides
  `GET /api/config`, and is named in a permanent 32px footer at the
  bottom of both dialogs that accept a secret. A startup banner is seen
  once by whoever started the process; a toast is seen once; a
  dismissible banner is the thing a user dismisses before doing the work
  it describes. The plaintext and memory cases render their caveat inline
  in a warning tone — a warning you must hover to discover is not one.

- `SecretStoreUnavailableError` is now the base every store's `set`
  throws, so the routes translate one condition into a 503 instead of
  matching `KeychainUnavailableError` by name.

Docs: the container section of the root README gains a table of which
store you get and how to make secrets survive, plus the
`MCP_INSPECTOR_SECRET_KEY` upgrade/mismatch rules.

Tests: the existing interface contract block is extracted to
`secretStoreContract.ts` and run against all three implementations, so
none can drift on the availability contract. `FileSecretStore` is tested
against a real temp filesystem — mode, cross-instance round-trip, that
the ciphertext does not contain the plaintext, the read-modify-write
race, and every refusal path. Full `npm run ci` passes, per-file gate
included.

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

clients/web/src/App.css

@@ -24,6 +24,13 @@
      light surfaces; a lighter red on the dark card/header (see dark override) —
      `red-6` fell just under WCAG AA on both. */
   --inspector-danger-text: var(--mantine-color-red-7);
+  /* Caution text (the secret-storage footer when secrets are unencrypted or
+     session-only, #1950). Yellow is the right *meaning* — this is "know what
+     you are doing", not "this failed" — but yellow is also the hardest hue to
+     make readable: `yellow-6`, the tone the log-level badges use on a filled
+     chip, is far under AA as body text on a light surface. `yellow-8` clears it
+     here, and the dark override lifts to `yellow-4` the same way danger does. */
+  --inspector-warning-text: var(--mantine-color-yellow-8);
 
   /* ── Surfaces ──────────────────────────────────────────── */
   /* Light-grey app background so the white cards read as raised. Drives
@@ -204,6 +211,9 @@
   --mantine-color-dimmed: var(--mantine-color-dark-1) !important;
   /* Lighter danger red for the dark card/header (deep red would fail AA). */
   --inspector-danger-text: var(--mantine-color-red-4);
+  /* Same inversion for caution text: the deep `yellow-8` is unreadable on a
+     dark surface. */
+  --inspector-warning-text: var(--mantine-color-yellow-4);
 }
 
 /* A11y contrast (light): Mantine sets these per-scheme on a selector more