Migrate tsconfig to Node16 module/moduleResolution

VS Code surfaced the TS deprecation:
  'moduleResolution=node10' is deprecated and won't work in TS 7.0.

Fix: switch the root tsconfig.json from
  module: CommonJS, moduleResolution: node
to
  module: Node16, moduleResolution: Node16

TypeScript 7 only supports node16/nodenext/bundler. node16 matches the
runtime semantics we already use (Node ≥ 16, CommonJS output via the
absence of "type": "module" in package.json), so the emit is unchanged.

Side effect of Node16 resolution: relative imports must carry the .js
extension. Added .js to every relative import across src/* (17 sites,
8 files). Bare module specifiers (express, electron, node:fs, ...) are
unaffected.

Verified:
- tsc -p tsconfig.server.json — 0 errors
- tsc -p tsconfig.installer.json — 0 errors
- node dist/server/app.js boots; /op login → 302, /op/list → 200

tsconfig.json

@@ -1,8 +1,8 @@
 {
   "compilerOptions": {
     "target": "ES2022",
-    "module": "CommonJS",
-    "moduleResolution": "node",
+    "module": "Node16",
+    "moduleResolution": "Node16",
     "outDir": "dist",
     "rootDir": "src",
     "esModuleInterop": true,