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.

Cargo.toml

@@ -1,5 +1,5 @@
 [workspace]
-members = ["server", "doctate-common", "client-desktop"]
+members = ["server", "common", "clients/desktop"]
 resolver = "3"
 
 [workspace.package]

clients/desktop/Cargo.toml

@@ -9,7 +9,7 @@ name = "doctate"
 path = "src/main.rs"
 
 [dependencies]
-doctate-common = { path = "../doctate-common" }
+doctate-common = { path = "../../common" }
 eframe = "0.28"
 reqwest = { workspace = true }
 serde = { workspace = true }