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>

CHANGELOG.md

@@ -5,6 +5,13 @@ All notable changes to RigDoctor are recorded here. Format follows
 (`MAJOR.MINOR.PATCH`, pre-1.0). `__version__` and `pyproject.toml` must match the git
 release tag (so the auto-updater, D18, can compare versions).
 
+## [0.3.0] - 2026-05-21
+### Added
+- **System inventory (M5)**: CPU, GPU (model/driver/VBIOS/VRAM/PCIe), motherboard/BIOS, RAM
+  (total + modules), storage, kernel, and display server. CLI `rigdoctor inventory`
+  (`--json` / `--markdown` / `--output`) and a GUI **Inventory** tab with Copy-as-Markdown,
+  Save, and "Run with admin" (for `dmidecode` board/BIOS/RAM details). Fills the last GUI tab.
+
 ## [0.2.0] - 2026-05-21
 ### Added
 - **"Check for updates" button** in the sidebar — force an immediate version check instead of