feat: add --cn flag to install from Gitee mirror (China)

[cavonlee]

Summary

Add a --cn flag that forces all downloads to the Gitee mirror, for users in mainland China where raw.githubusercontent.com is often unreachable.

Previously the installer framework had a GitHub-availability check with Gitee fallback, but it was unreliable: several URLs in install.sh were hardcoded to GitHub and bypassed the fallback entirely, and the framework itself is downloaded from GitHub before any check runs.

Changes

• Pre-scan args for --cn before downloading the installer framework (the framework URL itself must use the correct source)
• Switch INSTALLER_URL to Gitee when --cn is set
• Parameterize GIT_REPO / GIT_RAW_BASE / GIT_RAW_SEP / PIPOWER5_DTBO_URL (Gitee raw URL format requires a /raw/ segment)
• Replace all hardcoded GitHub URLs in install.sh with the new variables
• Fix install report label to show Gitee (--cn) when the flag is used

How to test

# Syntax check
bash -n install.sh

# Normal mode (GitHub) — unchanged behavior
curl -sSL https://raw.githubusercontent.com/sunfounder/pironman5/1.3.x/install.sh | sudo bash

# China mirror mode (Gitee)
curl -sSL https://gitee.com/sunfounder/pironman5/raw/1.3.x/install.sh | sudo bash -s -- --cn

# With variant
curl -sSL https://gitee.com/sunfounder/pironman5/raw/1.3.x/install.sh | sudo bash -s -- --cn --variant base

# Verify the install report shows "Source: Gitee (--cn)"
# Verify all downloads come from gitee.com (no raw.githubusercontent.com requests)

Verified: all 7 derived Gitee URLs (version.py fetches, installer framework, dtbo, HDMI EDID plugin) return 200; all 5 repos (pironman5, pipower5, pm_auto, pm_dashboard, sf_rpi_status) are cloneable from Gitee.