feat: docker run to compose converter (#623)

* feat(convert): harden /api/convert endpoint with auth, validation, and tests

Applies authMiddleware to the docker run to compose endpoint, validates
that the payload is a non-empty string within an 8192 character budget,
rejects inputs containing null bytes, and wraps composerize in a
try/catch that surfaces a 422 with a clear message when the library
cannot produce a services block. Adds a Vitest suite covering the auth
gate, happy path, common flag coverage, boundary and null byte
placement variants, and malformed command handling.

* feat(editor): add From Docker Run tab to create stack dialog

Introduces a third tab in the Create New Stack dialog that accepts a
docker run command, calls the converter endpoint, and previews the
returned compose YAML before writing it to a new stack directory. Uses
the defensive toast pattern, clears the stale preview when the input
changes, and rolls back the empty stack directory if saving the
converted YAML fails so the user never ends up with an orphan stack.

* docs(stack-management): document docker run to compose converter

Adds a Convert from a docker run command section to the stack
management page covering how to use the new tab, the list of supported
flags, and troubleshooting for unparseable inputs. Screenshots show the
empty tab, a successful conversion with the compose preview, the
resulting stack in the editor, and the error toast surfaced when the
input cannot be converted. Appends a matching entry to the
troubleshooting page.

docs/operations/troubleshooting.mdx

@@ -561,3 +561,20 @@ docker logs -f sencho
 ```
 
 The backend logs all route errors and service failures to stdout. This is the first place to look when the UI shows an error with no useful message.
+
+---
+
+## "Could not parse command" when converting docker run
+
+**Symptom:** The **From Docker Run** tab in the Create Stack dialog returns an error toast saying the command could not be parsed.
+
+**Cause:** The input is not a well-formed `docker run` command, or it uses a flag the converter does not recognize.
+
+**Fix:**
+
+- Make sure the command starts with `docker run` and ends with an image reference (e.g. `nginx:alpine`).
+- Check the [list of supported flags](/features/stack-management#supported-flags). Rare flags (such as `--userns` or custom runtime options) are not recognized. Remove the flag before converting and add it back manually in the editor afterwards.
+- Collapse multi-line commands into a single line. Mixed quoting across `\`-continued lines is a common source of parse errors.
+- Commands longer than 8192 characters are rejected. Trim anything non-essential and run the converter on the reduced command.
+
+When only part of a command is supported, convert what you can, paste the resulting YAML into the **Empty** tab as a starting point, and hand-edit the rest in `compose.yaml`.