feat(v0.8.8): linux ARM64 prebuilts + install docs overhaul

Triggered by a Telegram report from a Chinese user trying to deploy
DeepSeek TUI on a HarmonyOS ARM64 thin-and-light: `npm i -g deepseek-tui`
exited with `Unsupported architecture: arm64 on platform linux` because
v0.8.7 only published x64 Linux artifacts. They worked around it with
`cargo install`, but the README never documented that path for ARM users.

This PR closes that gap on three layers:

- **Release workflow** — add `aarch64-unknown-linux-gnu` to the build
  matrix using GitHub's `ubuntu-24.04-arm` runner. v0.8.8 will publish
  `deepseek-linux-arm64` and `deepseek-tui-linux-arm64` alongside the
  existing x64/macOS/Windows assets, plus add the row to the Release
  body's manual-download table.

- **npm wrapper** — uncomment the linux/arm64 row in `ASSET_MATRIX`,
  rewrite the `Unsupported architecture/platform` error to print the
  full `cargo install deepseek-tui-cli deepseek-tui --locked` recipe
  and link to docs/INSTALL.md, and add `DEEPSEEK_TUI_OPTIONAL_INSTALL=1`
  so CI matrices that include unsupported platforms can keep running
  without a binary.

- **Docs** — new docs/INSTALL.md covering every supported platform,
  prebuilt vs. cargo install vs. manual download, cross-compiling x64
  -> ARM64 with `cross` or `gcc-aarch64-linux-gnu`, China mirror setup,
  and a troubleshooting section for the common arm64, MISSING_COMPANION_BINARY,
  and self-update arch-mapping (#503) errors. README and README.zh-CN
  now have an explicit Linux ARM64 quickstart pointing at `cargo install`
  for v0.8.7 today and `npm i -g` for v0.8.8+; the v0.8.7 known-issue
  block is updated to mention both #503 and the missing arm64 prebuilt.

https://claude.ai/code/session_01Fg1FKMtDxVnC4pp6bNBRCS

.github/workflows/release.yml

@@ -54,6 +54,10 @@ jobs:
             target: x86_64-unknown-linux-gnu
             binary: deepseek
             artifact_name: deepseek-linux-x64
+          - os: ubuntu-24.04-arm
+            target: aarch64-unknown-linux-gnu
+            binary: deepseek
+            artifact_name: deepseek-linux-arm64
           - os: macos-latest
             target: x86_64-apple-darwin
             binary: deepseek
@@ -71,6 +75,10 @@ jobs:
             target: x86_64-unknown-linux-gnu
             binary: deepseek-tui
             artifact_name: deepseek-tui-linux-x64
+          - os: ubuntu-24.04-arm
+            target: aarch64-unknown-linux-gnu
+            binary: deepseek-tui
+            artifact_name: deepseek-tui-linux-arm64
           - os: macos-latest
             target: x86_64-apple-darwin
             binary: deepseek-tui
@@ -154,6 +162,7 @@ jobs:
             | Platform | Dispatcher | TUI runtime |
             |---|---|---|
             | Linux x64 | `deepseek-linux-x64` | `deepseek-tui-linux-x64` |
+            | Linux ARM64 | `deepseek-linux-arm64` | `deepseek-tui-linux-arm64` |
             | macOS x64 | `deepseek-macos-x64` | `deepseek-tui-macos-x64` |
             | macOS ARM | `deepseek-macos-arm64` | `deepseek-tui-macos-arm64` |
             | Windows x64 | `deepseek-windows-x64.exe` | `deepseek-tui-windows-x64.exe` |