feat(web): catalog vs read-only session launch model + apply --header (#1481, #1483) (#1496)

* feat(web): catalog vs read-only session launch model + apply --header (#1481, #1483)

Per specification/v2_catalog_launch_config.md, `mcp-inspector --web` now selects
the backend's server-list source and a session-wide writability, instead of
seeding an inert `initialMcpConfig` the UI never read:

- `--catalog <path>` (and `MCP_CATALOG_PATH`) → active WRITABLE catalog
  (seed/CRUD/watch), like the default catalog.
- `--config <path>` → READ-ONLY session file: served in the UI but never
  written, seeded, or plaintext-secret-migrated — safe to point at a foreign
  config (e.g. claude_desktop_config.json).
- ad-hoc `--server-url`/command (+ `--header`) → one server held IN MEMORY
  (no file written); `--header` is lifted onto the entry so the seeded
  connection actually uses it (closes the #1483 warn-only no-op).
- nothing → default catalog (writable), unchanged.

Backend (`createRemoteApp`): new `writable` + `initialServers` options. GET
/api/servers branches three ways (in-memory / read-only file with no
seed+no migrate / writable catalog); the four mutating routes return 403 when
read-only; `/api/config` carries `writable`; the file watcher is skipped for
in-memory sessions. `--catalog`/`--config` are mutually exclusive and neither
combines with an ad-hoc target or `--header`; `--header` requires an ad-hoc
HTTP/SSE server (clear errors otherwise — no silent no-op).

UI: `useServerListWritable` reads `writable` from /api/config; the Servers
screen hides add/edit/clone/remove/reorder/settings and shows a read-only
banner when false. All new props default to writable=true (back-compatible).

Docs: spec gaps G2/G3/G4 updated; launcher README documents the flags.

Verified end-to-end against the built launcher: --config (writable:false,
file byte-stable, mutations 403), ad-hoc --header (in-memory, header lifted),
and --catalog (writable:true, POST persists).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

* refactor(web): address review — Mantine <Code> in read-only banner, clarify watcher comment

- Replace raw <code> with Mantine <Code> in the read-only-session Alert
  (repo convention: prefer Mantine primitives over raw HTML).
- Add a comment near `watchable` clarifying that a read-only `--config` file
  is still watched on purpose (read-only blocks UI writes, not external
  observation; `writable` does not gate watching).

Deferred (per reviewer): consolidating useSandboxUrl + useServerListWritable
into one useConfig hook to avoid the duplicate /api/config fetch — optional
follow-up. The redundant ad-hoc header type-guard is kept as defensive.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

---------

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/launcher/README.md

@@ -10,6 +10,22 @@ The launcher is the package that provides the global `mcp-inspector` binary (e.g
 
 All configuration parsing, config-file loading, and server setup are handled by the app runners and by **core**; the launcher does not interpret config or env vars.
 
+## Web server-list flags (`--web`)
+
+`mcp-inspector --web` chooses which server list the UI shows and whether it is
+editable (see [specification/v2_catalog_launch_config.md](../../specification/v2_catalog_launch_config.md)):
+
+| Invocation | Server list | Editable in UI? |
+|------------|-------------|-----------------|
+| `mcp-inspector --web` | Default catalog `~/.mcp-inspector/mcp.json` | Yes |
+| `mcp-inspector --web --catalog <path>` (or `MCP_CATALOG_PATH=<path>`) | That file as the active catalog (created/seeded if missing) | Yes |
+| `mcp-inspector --web --config <path>` | That file as a **read-only session** — shown but never written, seeded, or migrated (safe for a foreign config) | No |
+| `mcp-inspector --web --server-url <url> --transport http --header "Name: Value"` (or a positional command) | One ad-hoc server held in memory, connectable with the given `--header`s | No |
+
+Rules: `--catalog` and `--config` are mutually exclusive; neither combines with
+an ad-hoc target or `--header`; `--header` requires an ad-hoc HTTP/SSE server
+and is applied to that connection (it is no longer a warn-only no-op).
+
 ## Publishing
 
 The root `@modelcontextprotocol/inspector` package ships as one fat tarball: `npm run build` at the repo root builds all clients, then `prepack` runs before `npm publish`. Runtime dependencies are declared on the root `package.json`; client builds bundle `@inspector/core` and externalize npm packages resolved from the root install.