refactor: split server and clients/desktop into standalone workspaces

Add [workspace] / [workspace.package] / [workspace.dependencies]
blocks to server/Cargo.toml and clients/desktop/Cargo.toml so each
becomes a self-contained workspace with its own Cargo.lock.

Remove the root Cargo.toml and Cargo.lock; the repo now hosts
three independent build worlds: server, clients/desktop, experiments.

Convert common/Cargo.toml to concrete versions (was workspace-
inherited from the root). common/ is consumed via path-dep from
all three worlds, but is not itself a workspace -- consumers
unify versions via their own lockfiles. Add common/Cargo.lock to
.gitignore per Cargo lib-only-crate convention.

Verification:
- server/Cargo.lock has 0 eframe/egui/winit entries
- cargo build --release --locked passes in server/
- test split: server 385 + desktop 89 + common 34 = 508 total
- bin output: clients/desktop/target/debug/doctate (named "doctate")

server/Cargo.toml

@@ -1,3 +1,20 @@
+[workspace]
+resolver = "3"
+
+[workspace.package]
+edition = "2024"
+version = "0.1.0"
+
+[workspace.dependencies]
+serde = { version = "1", features = ["derive"] }
+serde_json = "1"
+uuid = { version = "1", features = ["v4", "serde"] }
+time = { version = "0.3.47", features = ["local-offset", "parsing", "formatting", "macros"] }
+tokio = { version = "1", features = ["full"] }
+reqwest = { version = "0.12", default-features = false, features = ["json", "multipart", "rustls-tls"] }
+tracing = "0.1"
+toml = "0.8"
+
 [package]
 name = "doctate-server"
 version.workspace = true