Add dedicated raw panel for ProcessGit viewer

[rg4444]

Motivation

• The viewer previously reused Gitea's DOM to show raw source which hid the custom GUI when toggling and produced inconsistent "raw source" views.
• Creating a dedicated raw mount and loading the file from the raw endpoint makes the behaviour deterministic and prevents the viewer from being toggled away by Gitea's DOM.
• This simplifies the toggle logic and avoids fragile heuristics that attempted to locate external .file-view/pre elements.

Description

• Updated the template templates/repo/view_file_processgit_viewer.tmpl to render a dedicated raw panel (#processgit-raw-mount with #processgit-raw-pre) alongside the GUI mount.
• Updated web_src/js/features/processgitviewer/index.ts to select #processgit-raw-mount and #processgit-raw-pre, compute the primaryRawUrl from payload.entryRawUrl and payload.path, and add loadRawSource() which fetches the raw text using fetch(..., {credentials: 'same-origin'}).
• Removed the previous DOM probing logic that attempted to find Gitea's raw panel, and replaced it with controlled showGui() / showRaw() toggles that switch between mount and rawMount and call loadRawSource() when showing raw.

Testing

• No automated tests were run for this change.
• Changes were committed and verified via static inspection of the modified files.
• CI/test suite should validate runtime behaviour after deployment.

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Codex Task