feat(installer): add SystemCheckPage with blocking-error gating

Adds a new wizard page (positioned right after Welcome, FreshInstall only)
that auto-runs EnvironmentCheckService on entry and shows one row per check
with status icon, localized title/message, and hint+help-link on
failure/unknown. A "Recheck" button re-runs it, guarded against re-entrancy.

IInstallerPage gets a BlocksNavigation default member; WizardViewModel's
Next button now binds to CanGoNext, which the current page can veto (used
here while a check run is in flight or a blocking Error+Failed result is
present — Warnings and Unknown results never block). The summary line names
the blocking checks so a disabled Next is self-explanatory.

Wires up DI for the check pipeline (IProcessRunner, IPortOwnerResolver,
per-run ClaudeCliLookup) and adds the checks.* / installer.systemCheck.*
locale keys in en.json + de.json.

Visual appearance is NOT verified — needs a manual pass in the running
installer.

src/ClaudeDo.Localization/locales/en.json

@@ -481,6 +481,17 @@
       "registerMcp": "Register MCP server with Claude",
       "registerMcpHint": "Runs 'claude mcp add' so Claude can view and manage your ClaudeDo tasks. You can change this later."
     },
+    "systemCheck": {
+      "title": "System Check",
+      "subtitle": "ClaudeDo checks your system before installing.",
+      "recheck": "Recheck",
+      "summary": {
+        "running": "Checking your system…",
+        "ok": "Everything looks good.",
+        "warnings": "{0} warning(s) found.",
+        "blocking": "{0} problem(s) must be fixed: {1}"
+      }
+    },
     "paths": {
       "title": "Data Paths",
       "subtitle": "Configure where ClaudeDo stores its data.",
@@ -525,6 +536,40 @@
       "continueAnyway": "Continue anyway"
     }
   },
+  "checks": {
+    "git": {
+      "title": "Git",
+      "hint": "Install Git and make sure it is on PATH."
+    },
+    "gitIdentity": {
+      "title": "Git Identity",
+      "hint": "Set your git identity: git config --global user.name \"...\" and user.email \"...\"."
+    },
+    "ports": {
+      "title": "Ports",
+      "hint": "Free the port, or change it on the Worker page after install."
+    },
+    "writeAccess": {
+      "title": "Write Access",
+      "hint": "Choose a different install directory, or run the installer as administrator."
+    },
+    "claudeCli": {
+      "title": "Claude CLI",
+      "hint": "Install the Claude CLI and make sure it is on PATH."
+    },
+    "claudeVersion": {
+      "title": "Claude CLI Version",
+      "hint": "Update the Claude CLI to a newer version."
+    },
+    "claudeAuth": {
+      "title": "Claude Login",
+      "hint": "Run 'claude auth login' to sign in."
+    },
+    "permissionModeAuto": {
+      "title": "Permission Mode",
+      "hint": "Update the Claude CLI to a version that supports --permission-mode auto."
+    }
+  },
   "planning": {
     "conflict": {
       "windowTitle": "Merge conflict",