Narrow the two blind `except Exception` handlers

[tschm]

Subcategory: Error handling & API UX (+ linting)
Score: error handling 7 → 10

Both sites are deliberate best-effort degradations that log a warning and continue — the intent is right. The problem is that except Exception also swallows genuine bugs (typos, attribute errors) in the same block, turning them into a silent warning.

What to change

• src/pycharting/api/interface.py:252 — around webbrowser.open(chart_url); narrow to webbrowser.Error (add OSError if needed)
• src/pycharting/core/server.py:141 — around app.mount(... StaticFiles ...); narrow to OSError / RuntimeError

done when

ruff reports no BLE001 and both handlers name specific exception types while keeping the existing warn-and-continue behaviour.

Evidence

BLE001 Do not catch blind exception: `Exception`
   --> src/pycharting/api/interface.py:252:20

---

Surfaced by the Rhiza quality gates during the template bump to jebel-quant/rhiza@v1.2.2, which enabled the stricter ruff rule families (ANN, A, BLE, ARG) and TYPECHECKER=both. Gate status at time of filing: make fmt FAIL, make typecheck FAIL, make docs-coverage FAIL, test-layout FAIL; make deptry, make security, make test PASS (154 tests, 100% coverage).