chore(deps): dependency upgrades — react-router 7, eslint 10, TS 6, plugin-react 6; drop unused zod (v0.16.5-beta)

Applies the open dependabot PRs in one validated pass:

- react-router-dom 6.30 -> 7.18 (drop-in for our API surface; verified
  in-browser: login, sidebar routing, ?type= deep-links, legacy redirects)
- react + react-dom -> 19.2.7, axios -> 1.18.0, lucide-react -> 1.21.0
- actions/checkout v6 -> v7 across all workflows
- backend: eslint 10 + @eslint/js 10 (fix 5 new no-useless-assignment
  findings), typescript 6.0.3 (explicit types: ["node","jest"] in
  tsconfig — TS 6 no longer auto-includes @types), remove zod (unused)
- frontend: @vitejs/plugin-react 6

Not taken: ioredis 5.11.1 (#55) — bullmq pins ioredis to exactly 5.10.1
(including latest bullmq 5.79), so the bump nests a second ioredis and
breaks BullMQ connection types.

Validation: backend tsc/lint/build + 92 tests green; frontend tsc/lint/
build + 11 tests green; docker stack rebuilt and browser-verified.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

.github/workflows/ci.yml

@@ -15,7 +15,7 @@ jobs:
         working-directory: frontend
 
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
 
       - name: Set up Node.js
         uses: actions/setup-node@v6
@@ -44,7 +44,7 @@ jobs:
         working-directory: backend
 
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
 
       - name: Set up Node.js
         uses: actions/setup-node@v6
@@ -70,7 +70,7 @@ jobs:
     runs-on: ubuntu-latest
 
     steps:
-      - uses: actions/checkout@v6
+      - uses: actions/checkout@v7
 
       - name: Set up Node.js
         uses: actions/setup-node@v6