refactor: rename crates, modules, npm scope and build constants to Stashden

[BryanFRD]

Refs #245. Stacked on #247.

Crates stashden-{core,client,api,cli,app}, modules stashden_*, the @stashden/* scope, the STASHDEN_API_URL / STASHDEN_SOURCE_URL build defines, the OIDC flow cookie, test scratch names and the CI database. Commands quoted in the docs and the site follow, so no layer leaves a broken cargo run -p behind.

Deliberately unchanged:

• ferrflow.json's package name, which FerrFlow uses to find the previous tags;
• /var/lib/roxycloud and the declared volume in the image, and the roxycloud user and database in docker-compose.yml, which existing installs keep their data under;
• the Sonar project key, which lives on the Sonar side.

Cargo.lock carries the same packages at the same versions, only renamed. Tested: the whole workspace, API suites against Postgres, clippy, fmt, and the web and site builds.

[ferrfleet]

The rename itself is consistent: crates, imports, the npm scope, the build constants and their define entries, the CSP check and CI all line up. There's one blocking issue: the Dockerfile's WEB_ROOT no longer matches where the bundle gets copied. Also, renaming FLOW_COOKIE will fail any OIDC login that's in flight during the deploy. That only lasts as long as one flow and needs no change.

[ferrfleet]

08b71c8 fixes the WEB_ROOT mismatch. The README no longer names the unpublished image. Nothing blocking remains.