Add secure credential storage, sessions, and a launch screen

Login now persists properly rather than through the JSON file the test
harness was using.

  dr-plat            SecretStore trait plus a Secret Service backend.
                     Verified against the live GNOME Keyring: store,
                     retrieve, delete, confirm-gone all round-trip.
  Session/SessionStore   splits credentials from settings — the app
                     password goes to the keyring (FR-NC-2), while
                     server, login, chosen root and format selection are
                     ordinary config. A test asserts the credential never
                     appears in the config file.
  LaunchModel        the launch-screen state machine, testable without a
                     display server: sign in, approve in browser, choose
                     folder, tick formats, sign out.
  launch.slint       the screen itself, in its own file.

Absence of a secrets daemon is an explicit degraded mode, not a silent
fallback to plaintext — the screen says sign-in will not persist rather
than letting the user find out next launch. Android's Keystore backend
fails loudly for the same reason: a no-op store would look like it
worked and then lose the credential.

Two bugs caught by tests rather than by running it:

  - fail() after busy() signed the user out, because busy() had already
    discarded the session. A failed *scan* would have logged you out.
    Busy now carries the session.
  - normalise_server upgrades http:// to https:// rather than accepting
    it. NFR-SEC-3 requires TLS, and silently sending a credential in the
    clear is not a decision to make on the user's behalf.

launch.slint is not yet wired into app.slint. Calling slint_build::compile
twice replaces the generated module rather than adding to it, which broke
the other in-flight work on dr-ui; I reverted that immediately. Wiring it
needs an import inside app.slint, which is that work's file to change.

419 tests passing across ten crates.

core/dr-pipeline/src/lib.rs

@@ -34,6 +34,7 @@
 pub mod descriptor;
 pub mod framing;
 pub mod graph;
+pub mod lens;
 pub mod operation;
 pub mod ops;
 
@@ -42,8 +43,10 @@ pub use descriptor::{
 };
 pub use framing::{CropRect, Framing};
 pub use graph::{EditGraph, OpCapability, ParamCapability};
+pub use lens::{compose_warps, ComposedWarp, Warp};
 pub use operation::{
     compose, compose_with_framing, Affects, ComposedShader, Helper, Operation, Uniform,
+    RESERVED_UNIFORM_FIELDS,
 };
 
 #[cfg(test)]