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.

common/Cargo.toml

@@ -0,0 +1,13 @@
+[package]
+name = "doctate-common"
+version.workspace = true
+edition.workspace = true
+description = "Shared types and helpers between doctate server and clients"
+
+[dependencies]
+serde = { workspace = true }
+uuid = { workspace = true }
+time = { workspace = true }
+
+[dev-dependencies]
+serde_json = { workspace = true }