Add read-only repository viewer (UI + backend)

Implements a read-only repository viewer for remote Gitea repos and local clones. Adds UI/CSS for viewer panels, breadcrumb/branch controls, file table, code/Markdown preview, and readme rendering (frontend/css/components.css, frontend/js/app.js). Extends app state and wiring (state.js, app.js) with viewer actions, branch/content loading, local/remote navigation, and preview helpers (base64 decoding, markdown rendering, syntax highlighting, 256 KB preview limit). Adds Gitea API helpers to fetch repo branches and contents (frontend/js/gitea-api.js) and Tauri JS bindings for local repo operations (tauri-api.js). Implements Rust backend commands to list branches, tree entries, and file contents (with size/binary checks and helper utilities) and wires them into the Tauri command registry (src-tauri/src/lib.rs). Also updates README to mention the new read-only viewer.

frontend/js/state.js

@@ -9,6 +9,21 @@ const state = {
   cloneUrlInput: "",
   cloneDestinationInput: "",
   commitMessage: "",
+  viewer: {
+    source: "",
+    repoName: "",
+    repoPath: "",
+    cloneUrl: "",
+    defaultBranch: "",
+    branch: "",
+    branches: [],
+    path: "",
+    entries: [],
+    selectedFile: null,
+    readmeFile: null,
+    loading: false,
+    error: "",
+  },
 };
 
 export function getState() {