feat: Git workflow, Vim mode, and preview→editor SyncTeX

Add three core VISION features on top of the barebones MVP:

Git workflow (backend + frontend):
- Backend: simple-git wrapper with per-project repo isolation, init
  (idempotent .gitignore reconciliation, handles empty repos), status,
  log, commit (per-commit identity via -c flags), remote, push, pull
- gitFor() now refuses to run without the project's own .git, preventing
  accidental parent-repo detection when projects/ lives inside the
  typst-leaf worktree
- Frontend: GitPanel (branch, ahead/behind, inline commit, push/pull,
  remote prompt, collapsible log), SettingsPanel (git identity in
  localStorage), Ctrl+Shift+S focuses commit input
- Status/log refresh driven by gitVersion bumps (no polling) with
  cancellation guards across project switches
- Auto-init on status failure for pre-existing projects

Vim mode:
- Off-by-default toggle persisted to localStorage
- Dynamic monaco-vim import, status bar div, StrictMode-safe lifecycle
  via editorReady reset on unmount

SyncTeX (preview → editor):
- Handle tinymist/preview/scrollSource notification and
  window/showDocument request
- Cross-platform path normalization (Windows drive letters, native
  backslash paths, URL-decoded paths)
- Editor jump effect with Monaco selection/reveal/focus, clears on
  completion

Robustness fixes across review cycles:
- Cancellation guards on git status/log effects to prevent stale writes
- bumpGitVersion after auto-init so log viewer updates
- jsonOrThrow extracts structured backend errors, handles empty bodies
- SettingsPanel/GitPanel validate localStorage identity shape
- GitPanel clears log state on project switch
- File path URI encoding preserves path separators (segment-wise encode)
- tsconfig lib bumped to ES2021 for String.replaceAll

AGENTS.md: removed stale execa/monaco-languageclient references

PLAN.md: implementation plan for this chunk

typst-leaf-backend/src/routes/projects.ts

@@ -4,6 +4,7 @@ import path from "node:path";
 import { pathToFileURL } from "node:url";
 import { PROJECTS_ROOT } from "../config.js";
 import { projectDir, projectNameSchema } from "../paths.js";
+import { gitInit } from "../git.js";
 
 export const projectsRouter: ExpressRouter = Router();
 
@@ -45,6 +46,9 @@ projectsRouter.post("/:project", async (req, res) => {
         "utf8",
       );
     }
+    await gitInit(name).catch((e) =>
+      console.warn(`[projects] git init failed for "${name}":`, e.message),
+    );
     res.status(201).json({ project: name });
   } catch (err) {
     res.status(500).json({ error: (err as Error).message });