refactor: migrate monolithic crate to Cargo Workspace with Clean Architecture

Transform the single binary crate into a 9-crate workspace monorepo:

- Root Cargo.toml as [workspace] manager with resolver = "2"
- zesdex-entities: Domain entity types (session, settings, store, message, etc.)
- zesdex-utils: Pure utility functions (error, logger, pagination, slug, clipboard)
- zesdex-dto: Data Transfer Objects for LLM provider API communication
- zesdex-ipc: Unix-socket IPC layer (client/server/framing/protocol)
- zesdex-iam: Identity & Access Management (Clean Architecture: domain/application/infrastructure)
- zesdex-cms: Content Management (Clean Architecture: domain/application/infrastructure)
- zesdex-middleware: HTTP middleware (Auth, CORS, Rate Limiting)
- zesdex-libs: Composition root (AppContext, DB init, JWT, Argon2)
- zesdex-backend: Main binary entry point + seed/migrate binaries
- DevOps: Dockerfile, docker-compose, Nix (flake/shell/default), CI/CD updates
- Remove dead root src/ and src-misc/ directories

All crate re-exports maintain backward compatibility with original
crate::model::*, crate::dto::*, crate::ipc::* module paths.
Feature crates enforce strict layer separation: domain -> application
-> infrastructure with generic trait-based dependency injection.

crates/zesdex-backend/src/app/runtime/context/window.rs

@@ -0,0 +1,88 @@
+#![allow(dead_code)]
+//! Single source of truth for resolving the active model's context
+//! window size, replacing three copies of the same lookup that had
+//! drifted (`Action::Compact`, `spawn_turn`, and `view/status.rs` each
+//! had their own inline version — the status bar's copy additionally
+//! displayed "?" on no match instead of falling back like the other two,
+//! an inconsistency this unifies away).
+use crate::model::app_config::AppConfig;
+use crate::model::settings::Settings;
+
+/// Resolve the context-window size (in tokens) for the currently
+/// configured provider/model.
+///
+/// Flow: find the `ModelRole` whose `provider`+`model` match
+/// `settings` -> use its `context_window` if set -> otherwise fall back
+/// to `app_config.default_context_window`.
+///
+/// Return: always a concrete token count, never "unknown".
+pub fn resolve(app_config: &AppConfig, settings: &Settings) -> usize {
+    app_config
+        .model_roles
+        .values()
+        .find(|role| role.provider == settings.provider && role.model == settings.model)
+        .and_then(|role| role.context_window)
+        .unwrap_or(app_config.default_context_window) as usize
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use crate::model::app_config::ModelRole;
+
+    #[test]
+    fn resolves_context_window_from_matching_model_role() {
+        let mut app_config = AppConfig::default();
+        app_config.model_roles.insert(
+            "default".to_string(),
+            ModelRole {
+                provider: "zen".to_string(),
+                model: "deepseek-v4-flash-free".to_string(),
+                max_tokens: None,
+                context_window: Some(128_000),
+                temperature: None,
+            },
+        );
+        let mut settings = Settings::default();
+        settings.provider = "zen".to_string();
+        settings.model = "deepseek-v4-flash-free".to_string();
+
+        assert_eq!(resolve(&app_config, &settings), 128_000);
+    }
+
+    #[test]
+    fn falls_back_to_default_context_window_when_no_role_matches() {
+        let app_config = AppConfig::default();
+        let mut settings = Settings::default();
+        settings.provider = "nonexistent".to_string();
+        settings.model = "nonexistent-model".to_string();
+
+        assert_eq!(
+            resolve(&app_config, &settings),
+            app_config.default_context_window as usize
+        );
+    }
+
+    #[test]
+    fn falls_back_to_default_when_matching_role_has_no_context_window_set() {
+        let mut app_config = AppConfig::default();
+        app_config.model_roles.insert(
+            "default".to_string(),
+            ModelRole {
+                provider: "zen".to_string(),
+                model: "deepseek-v4-flash-free".to_string(),
+                max_tokens: None,
+                context_window: None,
+                temperature: None,
+            },
+        );
+        let mut settings = Settings::default();
+        settings.provider = "zen".to_string();
+        settings.model = "deepseek-v4-flash-free".to_string();
+
+        assert_eq!(
+            resolve(&app_config, &settings),
+            app_config.default_context_window as usize
+        );
+    }
+}