feat(updater): support custom GitHub release sources

[WilliamWang1721]

Summary

Add one optional custom GitHub Releases source to Settings → Application while keeping the existing updater pipeline unchanged.

• Leave the field empty to keep the official Kuddev/pebrel updater.
• .../releases or .../releases/latest checks that repository's latest release.
• .../releases/tag/<tag> checks that exact release, so fork builds and beta/preview releases can be selected.
• Existing update checks, background download, SHA-256 verification, native asset selection and platform install handoff are reused.

Design

This intentionally changes only the release-source boundary.

• Custom sources are restricted to HTTPS github.com/<owner>/<repo>/releases... URLs.
• The existing official 403/429 fallback remains unchanged.
• Custom sources do not add a second fallback path.
• Package naming, size limits, SHA-256 requirements, streamed verification and platform installation checks remain unchanged.
• Changing the source invalidates stale in-flight update checks and cancels the cached download session.

Scope

The implementation is intentionally small:

• one persisted update_release_url setting;
• one Settings input;
• one small release-source parser/validator;
• minimal wiring into the existing updater and download validation.

The existing fallback.rs and assets.rs remain identical to upstream main.

Validation

• Invalid-source and source-mismatch feedback use typed English and Simplified Chinese catalog entries.
• Current PR head: 4e47c5ca79a7627393daf1eb333b58a6a490fcdc (base 360613aa6eedfa4e441d658d98db502e8a81442b). Post-sync snapshot: architecture-contracts and lint pending; pr-size passed. Native Tests jobs were not reported. See live PR checks for current state.
• No local tests or builds were run for this follow-up.

AI assistance for this review and follow-up: GPT-6 Luna (max reasoning effort). Earlier implementation model metadata is unavailable in this continuation.

[Copilot]

Copilot review overview

🟡 Changes recommended

Unresolved findings affect stale update state, reset behavior, release-link validation, and localized validation errors.

Review effort: Lite
Findings: 3

Open (3)

• Background results ignore source generation · New
• About link bypasses release source validation · New
• Reset does not invalidate source updates or downloads · New

What changed in this PR

Adds a persisted custom GitHub Releases source while reusing the existing update and download pipeline.

Changes:

• Adds update_release_url with reset support and Settings UI.
• Implements GitHub release-source parsing and asset validation.
• Updates checks, downloads, release links, and localization.

File	Summary
nebula_settings/​src/​reset.rs	Registers the setting for reset; reset does not invalidate stale checks or cancel downloads (moderate, 4 votes).
nebula_settings/​src/​lib.rs	Adds persistence, defaults, and tests.
nebula_app/​src/​update_download.rs	Applies source-aware download validation.
nebula_app/​src/​update_check/​source.rs	Parses sources and validates assets; UI errors are hardcoded in Chinese (nit, 1 vote).
nebula_app/​src/​update_check.rs	Uses configured release sources.
nebula_app/​src/​gpui_shell/​workspace/​update_dialog.rs	Opens configured release pages.
nebula_app/​src/​gpui_shell/​settings_pane/​initialization.rs	Initializes the source input.
nebula_app/​src/​gpui_shell/​settings_pane/​about.rs	Adds source editing and links; background results can become stale after source changes (moderate, 4 votes), and links bypass source parsing (moderate, 2 votes).
nebula_app/​src/​gpui_shell/​settings_pane.rs	Stores and localizes source input state.
nebula_app/​i18n/​zh-CN.json	Adds Simplified Chinese strings.
nebula_app/​i18n/​en-US.json	Adds English strings.
architecture/​notes/​nebula_app/​update_check/​2026-09-25-custom-release-source.md	Documents the design decision.

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