feat: system inventory (M5)

CPU, GPU (model/driver/VBIOS/VRAM/PCIe), motherboard/BIOS, RAM (total +
modules), storage (real disks only), kernel, and display server.

- core/inventory.py: collect() + Markdown/JSON/text renderers + dict round-trip;
  stdlib + nvidia-smi/lspci/lsblk/dmidecode, all degrading gracefully
- cli: `rigdoctor inventory` (--json / --markdown / -o)
- gui: Inventory tab (fills the last empty tab) with Copy-as-Markdown, Save, and
  "Run with admin" (pkexec) for dmidecode board/BIOS/RAM details
- tests for collect/render/round-trip; remove unused placeholder-page helper

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "setuptools.build_meta"
 
 [project]
 name = "rigdoctor"
-version = "0.2.0"
+version = "0.3.0"
 description = "Modular hardware monitoring & crash diagnostics for Linux gamers."
 readme = "README.md"
 requires-python = ">=3.11"