Add terminal client, memory, model fallback, version and install improvements

- codecraft: full-screen terminal client (ratatui) – local tools and files,
  models via the CodeCraft server gateway or own providers (standalone with a
  guided setup), slash commands, @files, permission dialogs, resume, update
- Static Linux CLI builds (x86_64/aarch64, cargo-zigbuild) served by the
  server: /cli/install.sh and /cli/download/<target>
- Project memory (remember tool, .codecraft/memory.md) and personal memory
- Model fallback when a host is unreachable or a budget is used up
- Version/commit in UI and health, daily release check, "What's new" dialog
- PWA: install button, maskable icon, manifest id and screenshots
- Case-insensitive detection of connection errors for retries

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

core/src/config.rs

@@ -79,6 +79,9 @@ pub struct ModelConfig {
     /// Max. simultaneous requests to this model (None = unlimited).
     #[serde(default, skip_serializing_if = "Option::is_none")]
     pub max_concurrency: Option<usize>,
+    /// Model to use when this one's host is unreachable or its budget is used up ("provider/model").
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    pub fallback: Option<String>,
     /// Extra request body fields sent as-is (e.g. temperature, reasoning_budget_tokens).
     #[serde(default, skip_serializing_if = "serde_json::Map::is_empty")]
     pub options: serde_json::Map<String, serde_json::Value>,