refactor: rename to common/, clients/desktop, clients/wearos

Move three directories into their target topology:
- doctate-common/ -> common/
- client-desktop/ -> clients/desktop/
- watch/wearos/   -> clients/wearos/

Update doctate-common path-dependency in 3 Cargo.toml files
(server, clients/desktop, experiments) and the workspace
members list in the root Cargo.toml. Crate names unchanged
(doctate-server, doctate-common, doctate-desktop).

Workspace still in single-Cargo.lock form; isolation into
standalone workspaces follows in stage 3. All 508 tests pass,
experiments standalone-workspace also resolves the new path.

server/Cargo.toml

@@ -5,7 +5,7 @@ edition.workspace = true
 default-run = "doctate-server"
 
 [dependencies]
-doctate-common = { path = "../doctate-common" }
+doctate-common = { path = "../common" }
 axum = { version = "0.8", features = ["multipart"] }
 tokio.workspace = true
 reqwest.workspace = true